Mac app should be able to continue running in the background after being quit

[zacwest]

Is your feature request related to a problem? Please describe.
It's possible this could be keyed off whether there's a menu bar item or not, but I'm not entirely sure how obvious that would be.

Describe the solution you'd like
"Quit" doesn't terminate e.g. sensor updating, and the app keeps running in the menu bar. Clicking the menu bar item or opening the .app re-opens the app.

Additional context
It's possible to do this using the same activation policy that hides the dock item, so Quit just changes the activation policy to not show it in the dock, and re-opening returns the policy to what we expect it to be.

[zacwest]

Also worth considering: does this open up the app for abuse? We don't want Home Assistant to be, effectively, spyware that can be installed without the person on the machine knowing about it. Requiring that the Menu Bar Item be present to run without the Dock icon via this mode may be a good requirement.