Enable `globalSummon` / `quakeMode` to work even when there isn't a running Terminal

This is probably going to need the "headless monarch" work we theorized. Different parts to this:

* We've already got `startOnUserLogin` which starts the Terminal window. 
* Is any window the monarch then?
* Is the monarch a phoenix (when it's killed, who restarts it?)

I've had many more thoughts on the matter in the past, I just need to have somewhere to put them.