Add WindowFullscreenButtonHint.

Add WindowFullscreenButtonHint.

Adds the fullscreen button on Mac OS X. Has no effect
on other platforms. Regarding the name I dropped the
"Mac" prefix (like MacWindowToolBarButtonHint). Other
platforms can use and we don't want to accumulate "Mac"
API's.

Change logic in QCocoaWindow check for this flag instead
of using WindowsMaximizebutton.

Change-Id: I9f2db75e8e71b3da36777c7e48852eda3ce9d534
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>