Unset the cursor when an Item is unparented

It's possible for a cursor to get stuck if an item
gets deleted. QQuickItemPrivate::derefWindow sets
the cursorItem in QQuickWindowPrivate to 0, but
doesn't unset the cursor. This causes the cursor
to get stuck until the user hovers their mouse over
an item that has a cursor set.
Merge remote-tracking branch 'origin/st' into st