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.