appDisplay: Use the desktop file index for app searching

Rather than scanning all apps for searching, use Ryan's new desktop
file index and the glib support APIs for app searching instead of our
own system.

https://bugzilla.gnome.org/show_bug.cgi?id=711631

src/shell-app-usage.h

@@ -31,8 +31,8 @@ GSList *shell_app_usage_get_most_used (ShellAppUsage *usage,
                                        const char    *context);
 int shell_app_usage_compare (ShellAppUsage *self,
                              const char    *context,
-                             ShellApp      *app_a,
-                             ShellApp      *app_b);
+                             const char    *id_a,
+                             const char    *id_b);
 
 G_END_DECLS