Make better use of window state flags

Now that all top-levels have fullscreen geometry, we can utilize
window flags for semantic resizing. WinRT supports hidden, snapped,
filled, and fullscreen modes which logically correspond to minimized,
normal, maximized, and fullscreen. Through this mapping, existing
Qt APIs such as showFullscreen() can be used to break out of a snap.
Additionally, the application is able to use the window state flags to
determine its semantic viewport size.

Change-Id: Idc3fca24f413867f599768e9ffab09de0f199a5d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>