LibGUI+WindowServer: Add a GResizeCorner widget.

This widget is automatically included in GStatusBar, but can be added in
any other place, too. When clicked (with the left button), it initiates a
window resize (using a WM request.)

In this patch I also fixed up some issues with override cursors being
cleared after the WindowServer finishes a drag or resize.

Servers/WindowServer/WSCursor.h

@@ -8,6 +8,8 @@ enum class WSStandardCursor {
     IBeam,
     ResizeHorizontal,
     ResizeVertical,
+    ResizeDiagonalTLBR,
+    ResizeDiagonalBLTR,
 };
 
 class WSCursor : public Retainable<WSCursor> {