window: Don't mark the event as handled for odd event combinations

This is an accidental regression from 7a109a1. If we mark the event as
handled, then we *need* to set grab_op, or do some other sort of
behavior, since we have a grab.
events: Make the ungrabbed button press handling more bullet proof

Instead of returning a value based on whether or not we handled it, we
have this logic: either we have taken a grab on the window, in which
case we have a grab op and have handled it ourselves, or we did not take
a grab and *need* to replay the event to the window.

Handle this in events.c by checking the grab operation in the same way
that we check the other grab ops.
window: Filter out buttons when checking if the event is unmodified