#20 Additional rules for multiple devices

[Lars Friedrichs]

At the moment each device is tracked completely seperate from others. There should be an editable ruleset of what to do if
a) a certain number of devices in a group is online/away
b) one of a group is online/away which is a special case of a)
therefore we need
1. grouping of configs and an editor
2. a group ruleset editor
3. ruleset checking on every online/away event.