boxpointer: Use the -arrow-rise as padding instead of a magic number

boxpointer: Use the -arrow-rise as padding instead of a magic number

This makes sense if we want to follow what the following comment says:
// We also want to keep it onscreen, and separated from the
// edge by the same distance as the main part of the box is
// separated from its sourceActor

Using a magic number violates the "separated from the edge ...
separated from its sourceActor" part.

https://bugzilla.gnome.org/show_bug.cgi?id=682343