workspace: Extend builtin struts to screen edge when possible

Struts are defined in terms of screen edges, so expand the rectangles
we get via set_builtin_struts() accordingly. However we do want to
allow chrome on edges between monitors, in which case the expansion
would render an entire monitor unusable - don't expand the rectangles
in that case, which means we will only use them for constraining
windows but ignore them for the client-visible _NET_WORKAREA property.

https://bugzilla.gnome.org/show_bug.cgi?id=730527