With this patch, CEvents no longer stop at the target object, but will
bubble up the ancestor chain as long as CEvent::is_accepted() is false.
To the set accepted flag, call CEvent::accept().
To clear the accepted flag, call CEvent::ignore().
Events start out in the accepted state, so if you want them to bubble
up, you have to call ignore() on them.
Using this mechanism, we now ignore non-tabbing keydown events in
GWidget, causing them to bubble up through the widget's ancestors. :^)
This changes the behavior of the "is_checkable" flag on GAbstractButton
to only be about user interaction checkability. In other words, it now
only prevents the user from checking/unchecking the button, the code.
