Cocoa: p1 bug fix: revert use of subWindowStacking

This reverts commit 3c2373d7ea9bc91bb537c0725984d19ad0fbab01.
After finding yet another bug related to cocoa child windows
(QTBUG-17738), we have no other option than to admit it was a wrong
move to use the API in the first place. Had we only known how many
side-effects and hidden bugs it would introduce. The original problem
we tried to solve were the cases where a stays-on-top parent window
executed a modal child dialog. This child should always stay on top of
its parent, but Cocoa would insist on pushing the window down to the modal
window level upon activating/deactivating the application. Some window
systems will always stack a window child on top of the parent, while
others (X11) seems to be more selective on this issue. On Mac, we already
stack windows a  bit differently, thinking first and foremost on tool
windows.

Since this change is going into a patch release (which is debatable,
since this changes behaviour, but p1 is a p1), we choose to add in a
backdoor for those users who by chance depend on this behaviour. Setting
the env var QT_MAC_USE_CHILDWINDOWS=1 will give you the old code
path, but we plan to remove this for Qt-4.8.

Also, this patch does fix the original bug described above by overriding
the setLevel method in NSWindow, and refuse Cocoa to level down
stays-on-top modal windows.

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