allows to perform audio-only call

- add a new button inside the chatview for audio-only call.
- add ic_audio_only_call_black_24px.svg for the button.

note: this patch does not affect the behaviour. We still open the
      video page when the call is in progress.

Change-Id: I49afea09c572093ee370752df6df56c3ddda7eb1
Reviewed-by: Guillaume Roguez <guilaume.roguez@savoirfairelinux.com>
4 files changed