main: Add (hidden) support for static workspaces

The dynamic-workspaces key was introduced to allow us to opt out of
writing the num-workspaces setting (which is ignored with the dynamic
workspace management anyway), but there'll be some expectations that
the setting will have an effect on the UI.
It's actually not very hard to support, so here's to the graybeards ...

https://bugzilla.gnome.org/show_bug.cgi?id=671568