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.
