Fix overview sizing with multimonitor

Fix overview sizing with multimonitor

Previously, the overview BoxLayout was sized and positioned explicitly
using the primary monitor coordinates, as its parent was at 0,0 and
with a fixed layout manager. Now we use a bin layout, so we need to
position and size the stack actor, and set the overview boxlayout to
expand.
Take also the occasion to use a MonitorConstraint instead of handling
position and size manually.

https://bugzilla.gnome.org/show_bug.cgi?id=694969