I think it is a shame to waste the nice name "nexus"
just for a test facility; rather I've named our central
routing hub in the UI-Bus gui::ctrl::Nexus
So it makes sense to name the fake for unit testing
the test-nexus (we're not at nexus 5 yet)
...providing the standard implementation of UI-Bus connectivity.
It seems reasonable to place all of the UI-Bus implementation into
a single translation unit