Add inhibtor when wifi hotspot is active

Introduce a manager object that can track different systems and add/remove inhibitors as needed. As a first action add an inhibitor when we're acting as a wifi hotspot as both are common on mobile: quick suspend and use as hotspot:

$ systemd-inhibit 
WHO                          UID  USER PID    COMM            WHAT    WHY                                       
NetworkManager               0    root 917    NetworkManager  sleep   NetworkManager needs to turn off networks 
...
foo                          1000 foo  472926 phosh           sleep   WiFi hotspot active     

The new class is also a good seed for adding device dependent policy e.g.: when on device a that lacks feature b during suspend then add an inhibitor (see https://source.puri.sm/Librem5/OS-issues/-/issues/272). The feature list could live in gmobile.

Marked as draft as only mildly tested. It would be nice to only inhibit if there's also devices connected to the hotspot.

@Jarrah this could be a base for !1173 (comment 1630959)