Use libmutter-wm, and build a real gnome-shell binary

Build gnome-shell as a binary linked against libmutter-wm, instead of
a module to be loaded by libmutter-wm. Move the majority of
initialization-type stuff from gnome_shell_plugin_start() into main().

We still build libgnome-shell as a shared library, so that the linker
doesn't discard all the methods that are never called from C.

https://bugzilla.gnome.org/show_bug.cgi?id=641724

src/shell-global-private.h

@@ -8,8 +8,8 @@
 
 void _shell_global_set_plugin      (ShellGlobal  *global,
                                     MetaPlugin   *plugin);
-void _shell_global_set_gjs_context (ShellGlobal  *global,
-                                    GjsContext   *context);
+
+GjsContext *_shell_global_get_gjs_context (ShellGlobal  *global);
 
 gboolean _shell_global_check_xdnd_event (ShellGlobal  *global,
                                          XEvent       *xev);