rework pointer

Previously, coordinating where XrdController, XrdPointer and XrdPointerTip was updated was quite messy.

This MR moves a lot of the responsibility for positioning and showing/hiding the pointer components into XrdController. This happens in the controller directly when XrdClient polls a new controller pose from gxr and calls xrd_controller_update_pose_pointer (controller, &event->pose);, because the other components should not really be involved in how the pointer and pointer tip are visually represented.

Code for grabbing and dragging windows is also moved to XrdController, cleaning up the large and confusing _test_hover() function, however the the window grabbing and dragging is still initiated by the window manager, when XrdClient polls a new controller pose from gxr and calls xrd_window_manager_controller_updated (priv->manager, controller);, because the window placement and movement will probably concern the window manager in the future.