[OMEMO] Inconsistent OMEMO behavior for non-anonymous public rooms (non-invite-only)

Currently, OMEMO plugin allows the user to enable OMEMO in non-anonymous, non-members-only rooms, which contradicts To use OMEMO in a Groupchat, the Groupchat should be non-anonymous and members-only. error message.

1. Gajim 1.1.3 blocks OMEMO only in anonymous rooms. Other room options (such as members-only) are not checked at all, which is inconsistent with To use OMEMO in a Groupchat, the Groupchat should be non-anonymous and members-only. error message. As long as the room is non-anonymous, OMEMO could be enabled (but see 2).

2. Gajim 1.1.3 does not correctly handle room configuration change. If you create the room (which is anonymous by default) and change it to non-anonymous, you won't be able to enable OMEMO right away. You need to leave and join the room again, otherwise you'll get To use OMEMO in a Groupchat, the Groupchat should be non-anonymous and members-only. error message.

We should:

1. Improve plugin support for non-anonymous, non-invite-only rooms (edit error message, handle room configuration change) (better)

OR

2. Prevent enabling OMEMO support in non-anonymous, non-invite-only rooms (worse)

OMEMO in non-anonymous, non-invite-only rooms is supported in Pidgin and ChatSecure; not supported in Conversations.