Open 1:1 chat instead of private message when doubleclicking a participant in a non-anonymous group chat

Doubleclicking a participant in a non-anonymous group chat should open a 1:1 chat (instead of a private message).

Fixes #9422 (closed) and adds a setting in Preferences > Chat.

To quote docs.modernxmpp.org:

If real JIDs are known to all participants, it is preferable to use that for private communication to avoid confusion. Through-MUC PMs have the following disadvantages:

• Only work while connected to the group chat
• Do not interact well with multiple devices (e.g. not all of a recipient's devices may be in a group chat)
• Can cause confusion if talking to the same person through different views (e.g. if the person is already a contact in your roster, and you already have a chat open with them)

Additionally avoids weird behaviour like:

• Private Message sent through group chat to self
• OMEMO through Private Messages (Fixes gajim-plugins#307 (closed))