change API for CairoWidget::focus_handler

change API for CairoWidget::focus_handler

This functor/closure is responsible for stealing focus from any existing text entry (or whatever else may have focus)
when clicking on a CairoWidget or derived class.

The old implementation just gave focus back to the editor canvas. The new version walks up the widget packing
heirarchy to find a focusable parent (from the CairoWidget for which it is invoked). If no focusable parent
is found, it cancels keyboard focus in the toplevel window containing the CairoWidget