[Meta]Multistream support in conferences (multiple active videos)

The goal of this issue is to group information/observations to support multi-stream in video conferences. This means to have a conference with several participants active and/or one participant with multiple videos as the active participant.

Stack

https://review.jami.net/q/topic:multistream-conf-protocol

iOS

1. TODO support video split
2. TODO design UX to support multistream

Android

1. TODO support video split
2. TODO design UX to support multistream

TODO cleanup API:

• getConferenceDetails return conf->getVideoInput() which should be unused
• switchInput() is deprecated