Replace gdk_threads_add_idle* with g_idle_add()

Replace gdk_threads_add_idle* with g_idle_add()

The main GDK thread lock is not portable and deprecated.

The only reason why gdk_threads_add_idle() and
gdk_threads_add_idle_full() exist is to allow invoking a callback with
the GDK lock held, in case 3rd party libraries still use the deprecated
gdk_threads_enter()/gdk_threads_leave() API.

Since we're removing the GDK lock, and we're releasing a new major API,
such code cannot exist any more; this means we can use the GLib API for
installing idle callbacks.

https://bugzilla.gnome.org/show_bug.cgi?id=793124