Liblinphone now supports only two types of tchat:
- conference based tchat (objects ClientChatRoom and ServerChatRoom)
- minimal one-to-one tchat (object BasicChatRoom).

The following tchat objects have been deleted:
  - ProxyChatRoom
  - BasicToClientGroupChatRoom
  - ClientGroupToBasicChatRoom

1. Conference based tchat

This type of tchat is always linked ot a conference object:
- ClientChatRoom belongs to a ClientConference
- ServerChatRoom belongs to a ServerConference

Only chat specific functionality such as sending a message and ephemeral
capabilities are now handled in the tchat object whereas participant and
subject handling is taken care by the conference object.

These type of tchat object is no longer stored in the map of tchat held
by the core but only in the conference map through the pointer of the
conference they belong to

Conference parameters have been reworked. Now, common parameters to audio
video conferences and tchat are now stored in the ConferenceParams
class whereas tchat specific functionalities such as ephemeral messages
and backends are in the ChatParams class

2. Class changes

- Delete Local Audio Video Conference Event Handler
- Merge MediaConference::Conference and Conference
- Merge MediaConference::RemoteConference and RemoteConference
- Merge MediaConference::LocalConference and LocalConference
- Make ChatRoom and ChatRoomCbs as HybridObject
- Rename RemoteConference to ClientConference and LocalConference to ServerConference
- Rename ClientGroupChatRoom as ClientChatRoom and ServerGroupChatRoom as ServerChatRoom
- Make account as weak ptr in conference parameters

3. Bug fixes

- Do not create audio and video graphs when updating conference on the
  client side