boxPointer: Use the anchor point to fix problems with allocations

boxPointer: Use the anchor point to fix problems with allocations

Instead of setting the x/y position of the box pointer, which results
in a long change of workarounds for limitations of the Clutter
layout system, set the anchor point instead, which takes the
positioning out of the layout system.

The position is computed as a combination of the position computed
from the allocation and the box pointer's size, and an offset that
we tween when animating showing and hiding the box pointer.

https://bugzilla.gnome.org/show_bug.cgi?id=645744