Add extensionSystem

Consumer documentation will live at http://live.gnome.org/GnomeShell/Extensions

In terms of implementation; basically we load extensions from the well-known
directories.  Add a GConf key to disable extensions by uuid.  There is a new
option --create-extension for the gnome-shell script which takes a bit of
interactive input, sets up some sample files, and launches gedit.

No extensions UI in this patch; that will come later.

https://bugzilla.gnome.org/show_bug.cgi?id=599661

src/shell-global.h

@@ -43,6 +43,12 @@ ShellGlobal *shell_global_get (void);
 
 MetaScreen *shell_global_get_screen (ShellGlobal  *global);
 
+gboolean shell_global_add_extension_importer (ShellGlobal *global,
+                                              const char  *target_object_script,
+                                              const char  *target_property,
+                                              const char  *directory,
+                                              GError     **error);
+
 void shell_global_grab_dbus_service (ShellGlobal *global);
 
 typedef enum {