Major rework of application data structures and caching

Before, we looked up application data in several ways; the ShellAppSystem
exported just application ids (though it parsed the .desktop files internally),
and we'd create a Gio.DesktopAppInfo object (reparsing the desktop file again),
wrapping that inside a JavaScript AppInfo class, and finally the AppDisplay
would again parse the .desktop file to get the categories.

Also, to look up applications by id previously, we traversed the entire
menu structure each time.

Some qualities such as the NoDisplay flag were not easily exposed in the old
system.  And if we wanted to expose them we'd have to change several different
application information wrapper classes.

All in all, it was quite suboptimal.

The theme of this new code is basically "just use libgnome-menus".  We do
not call into Gio for app lookups anymore.  The new Shell.AppInfo class
is a disguised pointer for the GMenuTreeEntry item.

To fix the caching, we keep a simple hash table of desktop id -> ShellAppInfo.

src/shell-global.h

@@ -38,8 +38,6 @@ gboolean shell_clutter_texture_set_from_pixbuf (ClutterTexture *texture,
 
 GdkPixbuf *shell_get_thumbnail(const gchar *uri, const gchar *mime_type);
 
-GSList *shell_get_categories_for_desktop_file(const char *desktop_file_name);
-
 guint16 shell_get_event_key_symbol(ClutterEvent *event);
 
 guint16 shell_get_button_event_click_count(ClutterEvent *event);