GTK3 backend: Replaced deprecated GObject calls with GLib

Using GObject.timeout_add and GObject.source_remove are deprecated and should be replaced by corresponding GLib calls.
Running a distro with an unstable PyGObject version will throw deprecation warnings for each call, so this patch fixes those. Run a GTK3 animation example with Python and the -Wd flags on stable versions to show these warnings.

This will *not* break any backwards compatibility.