boxpointer: Use X/Y actor positioning instead of an anchor point

boxpointer: Use X/Y actor positioning instead of an anchor point

Currently all the widgets using BoxPointer and so popup actors are always drawn
at screen origin and then shifted at paint time in the proper position.

This doesn't work when using resource scale, since the widgets need to know
in which monitor they currently are in order to use properly scaled resources.

So, basically revert commit 22c22e0d7 and go back using the actual actor
coordinates for positioning and the actor translation for animating it, as the
relayouting issues of the past seem to be gone now.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/1008
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/435