boxes: Ignore struts that don't attach to the side they claim

Like the _NET_WM_STRUT/_NET_WM_STRUT_PARTIAL client properties,
_NET_WORKAREA is defined in terms of screen geometry rather than
taking individual monitors into account. However we do want to
allow system chrome to be attached to a monitor edge rather than
a screen edges under some circumstances. As not all clients can
be assumed to deal gracefully with the resulting workarea, use
those "struts" only internally for constraining windows, but
ignore them when exporting _NET_WORKAREA.

https://bugzilla.gnome.org/show_bug.cgi?id=730527
testboxes: Update test cases

Who cares? We do now ...

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