widget: Remove parent-set signal

widget: Remove parent-set signal

When a widget unparents its child widget manually in finalize, this can
lead to the parent-set signal being emitted for those child widgets. The
parent already has a ref_count of 0 though, so it can't be used in a
meaningful way. Specifically, emitting the signal will already try to
ref the parent which prints a critical.

Since GtkWidget already has a "parent" property, one can use its notify
signal instead to get notified when the parent widget changes.