Multiple Account Support

Allow using multiple accounts simultaneously.

This requires a major redesign in userId/token saving, autologin and room list model. The benefit will be much clearer backend logic, fewer crappy codes, and multiple accounts.

• Replace image provider in room list and side bar to support fetching avatars from different QMatrixClient::Connection*
• Change backend to allow a list of Connection* instead of one single Connection*. This at least affects RoomListModel.
• Change Controller's credential storing mechanism to allow storing multiple credentials. Controller is no longer a wrapper above a single Connecton*.
• Change auto-login mechanism.
• Logout should not exit the app.