QCocoaWindow - do not call resignKeyWindow

QCocoaWindow::setMouseGrabEnabled/setKeyboardGrabEnabled calls
resignKeyWindow. According to Apple's docs, we should _never_ call
this function (it's done by Cocoa automatically).
Calling this function leaves a window in weird state where it can
not become key again until you explicitly make another window key first.

Task-number: QTBUG-53050
Change-Id: I7a887659df8df11880328ffa2adc07c4a3af63e6
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>