CI: add uncrustify config.

Run in the repo's root directory with for example

```
$ uncrustify -c data/uncrustify.cfg src/**/*.c
```

data/uncrustify.cfg

@@ -0,0 +1,112 @@
+# Enorces he code style of Mutter (based on GNU)
+
+code_width                               = 80
+
+# indent using tabs
+output_tab_size                          = 2
+indent_columns                           = output_tab_size
+indent_with_tabs                         = 0
+indent_brace                             = 2
+
+# indent case
+indent_switch_case                       = 0
+# indent_case_brace                        = 0
+
+# newlines
+newlines                                 = lf
+nl_after_semicolon                       = true
+nl_start_of_file                         = remove
+nl_end_of_file                           = force
+nl_end_of_file_min                       = 1
+
+# spaces
+sp_return_paren                          = force      # "return (1);" vs "return(1);"
+sp_sizeof_paren                          = force      # "sizeof (int)" vs "sizeof(int)"
+sp_assign                                = force
+sp_arith                                 = force
+sp_bool                                  = force
+sp_compare                               = force
+sp_after_comma                           = force
+sp_case_label                            = force
+sp_else_brace                            = force
+sp_brace_else                            = force
+sp_func_call_paren                       = force      # "foo (" vs "foo("
+sp_func_proto_paren                      = force      # "int foo ();" vs "int foo();"
+sp_before_ptr_star                       = force
+sp_after_ptr_star_qualifier              = force      # "const char * const" vs. "const char *const"
+sp_after_ptr_star                        = remove
+sp_between_ptr_star                      = remove     # "**var" vs "* *var"
+sp_inside_paren                          = remove     # "( 1 )" vs "(1)"
+sp_inside_fparen                         = remove     # "( 1 )" vs "(1)" - functions
+sp_inside_sparen                         = remove     # "( 1 )" vs "(1)" - if/for/etc
+sp_after_cast                            = force     # "(int) a" vs "(int)a"
+sp_func_call_user_paren                  = remove     # For gettext, "_()" vs. "_ ()"
+set func_call_user _ N_ C_                            # Needed for sp_after_cast
+sp_before_semi                           = remove
+sp_paren_paren                           = remove     # Space between (( and ))
+
+eat_blanks_before_close_brace            = true
+eat_blanks_after_open_brace              = true
+
+# Style for curly braces
+nl_assign_brace                          = add
+nl_enum_brace                            = add
+nl_union_brace                           = add
+nl_struct_brace                          = add
+nl_class_brace                           = add
+nl_do_brace                              = add
+nl_if_brace                              = add
+nl_for_brace                             = add
+nl_else_brace                            = add
+nl_elseif_brace                          = add
+nl_while_brace                           = add
+nl_switch_brace                          = add
+nl_fcall_brace                           = add
+nl_fdef_brace                            = add
+nl_brace_else                            = add
+nl_brace_while                           = add
+nl_case_colon_brace                      = add
+nl_after_brace_open                      = true
+
+# Function calls and parameters
+nl_func_paren                            = remove
+nl_func_def_paren                        = remove
+nl_func_decl_start                       = remove
+nl_func_def_start                        = remove
+nl_func_decl_args                        = ignore
+nl_func_def_args                         = ignore
+nl_func_decl_args_multi_line             = force
+nl_func_def_args_multi_line              = force
+nl_func_decl_end                         = remove
+nl_func_def_end                          = remove
+
+# Code modifying options (non-whitespace)
+mod_full_brace_function                  = force
+mod_remove_extra_semicolon               = true
+
+# Align
+align_func_params                        = true
+align_single_line_func                   = true
+align_var_def_star_style                 = 2
+
+# one liners
+nl_func_leave_one_liners                 = true
+nl_enum_leave_one_liners                 = true
+nl_assign_leave_one_liners               = true
+
+# Comments
+cmt_cpp_to_c                             = true       # "/* */" vs. "//"
+cmt_convert_tab_to_spaces                = true
+#cmt_reflow_mode                         = 2          # Full reflow (seems doesn't work quite well, it doesn't reorder the comments)
+cmt_width                                = 80         # Line width
+cmt_star_cont                            = true       # Whether to put a star on subsequent comment lines
+cmt_sp_after_star_cont                   = 1          # The number of spaces to insert after the star on subsequent comment lines
+cmt_c_nl_start                           = false      # false/true
+cmt_c_nl_end                             = true       # false/true
+# For multi-line comments with a '*' lead, remove leading spaces if the first and last lines of
+# the comment are the same length. Default=True
+cmt_multi_check_last                     = false
+
+# Encoding
+utf8_bom                                 = remove
+utf8_force                               = true