Make color constants work without warnings

The code for defining a color as a constant had broken logic: it
would try to parse the color first as an double, then as an integer;
the second attempt would produce an error about overwriting the
already-set-GError. Then it would clear the error and store the constant
as a color.

Use the fact that colors have to start with a letter or #, divide the
space of constants into:

 - Integers
 - Doubles
 - Colors

so we get good error messages. Based on a patch by
William Jon McCann <jmccann@redhat.com>.

Note that this breaks the ability to specify an integer constant as
identical to another integer constant (the same didn't work for doubles.)
I think this was an accidental side effect of the code and not something
that was intentional or people were relying on

https://bugzilla.gnome.org/show_bug.cgi?id=632116

src/ui/theme-parser.c

@@ -902,9 +902,20 @@ parse_toplevel_element (GMarkupParseContext  *context,
                               NULL))
         return;
 
-      if (strchr (value, '.') && parse_double (value, &dval, context, error))
+      /* We don't know how a a constant is going to be used, so we have guess its
+       * type from its contents:
+       *
+       *  - Starts like a number and contains a '.': float constant
+       *  - Starts like a number and doesn't contain a '.': int constant
+       *  - Starts with anything else: a color constant.
+       *    (colors always start with # or a letter)
+       */
+      if (value[0] == '.' || value[0] == '+' || value[0] == '-' || (value[0] >= '0' && value[0] <= '9'))
         {
-          g_clear_error (error);
+          if (strchr (value, '.'))
+            {
+              if (!parse_double (value, &dval, context, error))
+                return;
 
               if (!meta_theme_define_float_constant (info->theme,
                                                      name,
@@ -915,9 +926,10 @@ parse_toplevel_element (GMarkupParseContext  *context,
                   return;
                 }
             }
-      else if (parse_positive_integer (value, &ival, context, info->theme, error))
+          else
             {
-          g_clear_error (error);
+              if (!parse_positive_integer (value, &ival, context, info->theme, error))
+                return;
 
               if (!meta_theme_define_int_constant (info->theme,
                                                    name,
@@ -928,10 +940,9 @@ parse_toplevel_element (GMarkupParseContext  *context,
                   return;
                 }
             }
+        }
       else
         {
-          g_clear_error (error);
-
           if (!meta_theme_define_color_constant (info->theme,
                                                  name,
                                                  value,