Bug 912494: conference call detail window

Briefing and known issues:
* Added a "groups-show" button to expose conference call detail.
  - Listener of `#groups-show` and `#calls` shouldn't be triggered at the same time so I excluded the case in listener of `#calls`. Thinking about better solutions..
* Added hangup button of individual calls in conferences.
 - Not sure if adding listener in `handledCall` is ok... ?
* "XXX debugging purpose only" code is still included for triggering conference call. I'll remove them when merging.
* Modified DOM structure of `group-call-details` to fit structure of Action menu.
* Refactored a `overlay.css` that can be used on both suggestion bar and conference call detail.
* (Follow-up issue) Still need to listen to `conferenceGroup.oncallschanged` to manage `HandledCall` correctly. If you hold any phone after exiting a conference call, some error happens.