boxpointer: If available, use source actor for constraining

boxpointer: If available, use source actor for constraining

If a source actor is set, use that for determining the arrow side (i.e.
whether the BoxPointer widget should expand in a certain direction).
This is better because it ensures that the popup is displayed on the
same monitor as the widget it originates from.

Without this, entering text with a vertically aligned input method
close to the bottom of a monitor would expand the BoxPointer downwards
on the monitor beneath it, instead of upwards, which is what one would
expect.

https://bugzilla.gnome.org/show_bug.cgi?id=786886