feature/question: area occupation sensors

**Is your feature request related to a problem? Please describe.**
I want to know if at least a single device is in an area.

**Describe the solution you'd like**
Per-Area occupancy sensor (true/false), optionally with a list of devices currently in that area and a "cooldown time".

**Describe alternatives you've considered**
It would probably be possible to do this with automations and helpers "by hand", but that is no elegant solution.

Not too sure if this is still in scope for bermuda though.