Improve maximized split-view behavior

Maximized views must be un-maximized before certain actions:
 - detach view
 - split view
 - move to new tab
 - close

This commit fixes the toggle state getting out of sync after those
actions, adds the un-maximize step for splitting a maximized view,
and reduces code duplication by adding ViewSplitter::clearMaximized(),

For now, clearMaximized() does need to be called manually: we want the
display restored before the removal, but ViewSplitter handles removals
after the fact with QEvent::ChildRemoved.