This replaces the previously hardcoded calls to gdk_window_set_user_data,
and also lets us track which windows are a part of a widget. Old code
should continue working as is, but new features that require the
windows may not work perfectly.
We need this for the transparent widget support to work, as we need
to specially mark the windows of child widgets.
https://bugzilla.gnome.org/show_bug.cgi?id=687842
We don't get an automatic queue resize on realize anymore, which
was papering over this bug where we did not set the child window
size/position at realize time.
When we're allocating children of GtkOverlay, compare their allocation
with the overlay one, and set left/right/top/bottom style classes if the
overlaid widget touches one or more of the overlay edges.
https://bugzilla.gnome.org/show_bug.cgi?id=669342
gtk_widget_translate_coordinates() can fail in case the widget is not
realized or there's no common ancestor. Don't use the x/y values
returned by that method in that case, since their value is undefined.
Move code for child allocation into the child allocation function. Don't
keep it in the overlay allocation code.
See the next commit for why this is useful.
