GtkWidget: Avoid lingering clock frame updates

GtkWidget: Avoid lingering clock frame updates

For some widgets, like GtkTreeView, which setup a clock frame
update during realize, it was possible to call
gdk_frame_clock_begin_updating() twice, but only ever disconnecting
from it once. This happens because the realized flag is set at an
unpredictable time by the GtkWidget's realize implementation.

Keep the signal handler ID from us connecting to the "update" signal
to avoid connecting to it twice.

This fixes high wake-up count from any application using GtkTreeView,
even idle ones.

https://bugzilla.gnome.org/show_bug.cgi?id=710666