Ensure GtkWidget::style-updated is emitted before first size negociation

Ensure GtkWidget::style-updated is emitted before first size negociation

Fixes Bug 639584 - initial emission of GtkWidget:style-set is
not happening.

GtkWidget was filtering out ::style-updated (and ::style-set)
emissions until the widget was realized in order to avoid often
useless updates during widget construction and placing.

This is now done instead until the widget has a parent/screen,
which ensures the initial emission happen prior to the first
size negociation, while still filtering out all early emissions
during construction.