Previously, we tried to move the context's origin to the widget's top
left location, no matter what window the paint was happening on. Now we
only do that for child windows of the widget's window and leave the
context untouched for windows that the widget has created outside its
own hierarchy. In those casses, we also don't clip the context to
the widget's allocation.

Includes fixes to GtkHandlebox for this effect and fixes all known
issues with it.

These semantics assume that gtk_widget_draw() should only draw the parts
of a widget that are inside child windows and not draw stuff that is
located in completely different GdkWindows. In the handlebox case, it
means that it should only draw the handle when it is attached, but not
when it isn't. We'll likely need a special draw function for the
detached handlebox if we want to draw it.