This reverts commit 30a1a79322. This turns out to be unnecessary when you can set the titlebar on the window and let GtkWindow handle the events. As a benefit, we get the window menu on custom titlebars for free.
30a1a79322
We use the same window-dragging style property here to enable this as for menubars and toolbars.
This widget has been developed as GdHeaderBar in libgd. The copy here has been renamed and changed to use GTK+ internals.