Allow windows to be dragged by clicking on empty areas in menubars
and toolbars. This is under theme control, via the GtkWidget::window-dragging
style property. The idea is that it makes sense to turn this on if a
theme makes the window frame and the menubar/toolbar appear visually
contiguous.

The main patch was written by Cody Russell, with a contribution by
Ayan George. See bug 611313.