Add gtk_widget_add_tick_callback(), remove GtkTimeline, etc.
Add a very simple GtkWidget function for an "tick" callback, which is connected to the ::update signal of GdkFrameClock. Remove: - GtkTimeline. The consensus is that it is too complex. - GdkPaintClockTarget. In the rare cases where tick callbacks aren't sufficient, it's possible to track the paint clock with ::realize/::unrealize/::hierarchy-changed. GtkTimeline is kept using ::update directly to allow using a GtkTimeline with a paint clock but no widget.
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