monitor-manager: Primary monitor fixes

When in single display mode switching monitors to the other display so far involved enabling it and then moving the primary display over. This fixes things so one can switch back and forth between both without the (non obvious) intermediate stop.

Technically this needs !939 (merged) but since it addresses a separate issue I've split this into a separate series.

I see two remaining issues:

• phoc#212 (closed) which I'll intend to look at once phoc!310 (merged) and its dependencies are in since otherwise there's too much churn atm (and there's overlap with the gesture code (see phoc!312 (merged)))
• making the idle manager aware of monitors going away so we dont' get an accidental blank when replugging (which I'll file as follow up once !939 (merged) is merged (it's not a new issue either but becomes easier to reproduce now)