video_mixer: support multiple video layouts in conference

This patch aims to improve the conference management for the host.
Now, the host is able to switch between 3 conferences layout:
1. The grid view (actual one) where all participants are shown
at the same height/width
2. The One big/Other in small which show one participant bigger than
the others
3. One participant in big

The daemon's API got two new methods:
+ setConferenceLayout() to switch between these layouts
+ setActiveParticipant() used in the 2 last layouts.

Change-Id: I3c16569e24d1b63331ffe9d79e35790a6ac47a0c