Mark window type properties as CONSTANT

Since we don't have any means of getting notified by _NET_WM when a
windowType changes, and since KWin would handle that quite poorly, AND
since no one does that in real-world applications anyway, we might as
well mark those getters as CONSTANT, so that QML engine would stop
complaining about depending on non-NOTIFYable properties.

According to the specification, the window type should be decided before
a window gets mapped.