Cocoa: Add QCocoaWindowPointer

This patch extends 5b54c352


by abstracting the watcher pattern. The class is specialized
for QCocoaWindow since it's aware of the QObject sentinel there.

We update the usage in QNSWindowHelper and extend it to the
forward window (this one is used for mouse coordinate conversion
when docking windows).

Change-Id: I628415527593daec835bbad1b6e83d13fe7b6703
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>