Cocoa: Improve native view lifetime accuracy.

Ideally all native NSWindows and NSViews owned by
QCocoaWindow should be deallocated during the QCocoaWindow
destructor. In reality this does not always happen
since Cocoa is free to hold references to the views
after Qt releases its reference.

We can help Cocoa clean up:

- Clear the first responder for the NSWindow under
  the ~QCocoaWndow() autoreleasepool.

- Use an autoreleasepool to clean up temp objects
  from [NSWindow orderFront:] immediately.

Together this makes the QNSView lifetime be contained
by the QCocoaWindow lifetime, at least for simple
QWindow usage. It also fixes the observed memory leak
reported in QTBUG-51766

Change-Id: Idd224f54ebd6f61f274461a204ff30c666b22768
Task-number: QTBUG-51766
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>