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
030e6aa507