Implement virtual method setWinId from base class QPlatformWindow

With modifying the base class for providing the setWinId method we don't need to install
the qweboswindow.h header file anymore for use in the webappmanager to provide just the
window id. Webappmanager can now use the QPlatformWindow class to provide the window id.

Signed-off-by: Simon Busch <morphis@gravedo.de>