widget: Add a frameclock_connected boolean

... and use it to not connect anything to the frameclock if it isn't
set.

This gets around the problem that the frame clock is disconnected before
GtkWidgetClass.unrealize() is called but the widget is still marked as
realized and the frame clock is available during the vfunc, which makes
calls like gtk_widget_queue_resize() reconnect to the frame clock.

Closes #168
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00:07:11