Use change signals on Window interface class.

QWaylandLayerSurface pulled data from the Window on startup. The Window
pushed data into the QWaylandLayerSurface on changes. Having two
patterns is a sign of something being off.

This moves everything to a single design, pulling from the public
interface. This allows us to drop a code path that meddles with
QWaylandWindow internals.

src/interfaces/window.h

@@ -93,6 +93,13 @@ public:
      */
     static Window *get(QWindow *window);
 
+Q_SIGNALS:
+    void anchorsChanged();
+    void exclusionZoneChanged();
+    void marginsChanged();
+    void keyboardInteractivityChanged();
+    void layerChanged();
+
 private:
     Window(QWindow *window);
     QScopedPointer<WindowPrivate> d;