Fix informing the underlying widget when leaving SplitterProxy

While the SplitterProxy is active, it intercepts all relevant events, so that
the underlying widget still thinks it's in the same "on splitter" state. When
the SplitterProxy is left, the underlying widget is sent a HoverLeave/HoverMove
event to make it aware of the new current cursor position. Without this, it
doesn't know that it's not supposed to be in the "on splitter" state, and when
it regains focus it just re-activates the SplitterProxy at the current cursor
position.

This was broken by accident in d201a1f1 ("Fix SplitterProxy not clearing
when above another QSplitterHandle"), which moved the hide() call past the
call to QCoreApplication::sendEvent. Previously that made isVisible() false,
which also prevented the interception of the HoverLeave/HoverMove events.

BUG: 436473


(cherry picked from commit f99b7ef6)