Add QPlatformWindow::setWindowModified(bool)

Add QPlatformWindow::setWindowModified(bool)

Mac OS X provided a platform API for marking a window/document as
modified. This previously worked in Qt 4, so we need to have an
interface to keep this working in Qt 5. If the platform window does not
provide an implementation, fallback to setting the window title.

Note that this does not add any QWindow API, it's only in
QPlatformWindow.

Change-Id: I84c5a5df8536859157f2b1fa9e4cc647a09fd06d
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>