Ignore child event types we're not explicitly interested in.

MouseEventListener listens to both child events and events passing
through itself; child events are recorded so the handler for the
latter can perform a comparison and avoid emitting signals for the
same event again. However, this comparison could fail because the
member used to record the last child event would also be updated
for events we were not actually interested in.

A real-world example of this is opening a popup menu in repsonse to
a Press event. This causes an Ungrab event on the child, which would
cause the comparison to fail and mousePressEvent to announce the same
press yet again.