Add g_desktop_app_info_search()

The first time this function is called we load all of the keyfiles in
the directory, ignoring the 'Hidden' ones and build an index out of the
interesting fields using g_str_tokenize_and_fold().

We do prefix matching on the tokens to find relevent desktop files.

Right now this is implemented as a hashtable that we iterate over,
checking prefixes on each token.  This could possibly be sped up by
creating an array, but it's already pretty fast...

https://bugzilla.gnome.org/show_bug.cgi?id=711557

gio/gdesktopappinfo.h

@@ -164,6 +164,8 @@ gboolean    g_desktop_app_info_launch_uris_as_manager (GDesktopAppInfo
 						       gpointer                    pid_callback_data,
 						       GError                    **error);
 
+GLIB_AVAILABLE_IN_2_40
+gchar *** g_desktop_app_info_search (const gchar *search_string);
 
 G_END_DECLS