Additional properties for Group. This would let any plugin include their own custom properties if needed.
HasInterfaces provides a method on_interface_plugged() to be overwritted in plugins if wanted to do something when an interface is plugged
Interfaces has on_plugged pure virtual method to be overwritten in plugins with initial code when plugged to a HasInterfaces object
NOTE: This branch should be landed along with https://code.launchpad.net/~ningbo-team/ningbo/allow-several-passive-sessions/+merge/305495.
Approved by Tiago Salem Herrmann, system-apps-ci-bot.