swarm: add call buttons and interactions for multi-swarm

+ Add call buttons to start a new call
+ React to events from the swarm
+ call interactions (Join call/Call ended, etc)
+ active calls area
+ Add call management logic in LRC
+ Feature is enabled via the experimental checkbox

jami-daemon#312
Change-Id: I83fd20b5e772097c0792bdc66feec69b0cb0009a