Support for chat markers in MUCs

"Displayed" chat markers for MUCs do not seem to be in use in gajim.

They serve 2 distinct features imho:

1. Knowing who in a MUC has read your messages

This is nice to make sure that your messages are actually read by someone, especially in a context (mobile clients) where presence information is often irrelevant.

2. Synchronizing your 'read history' between clients

I believe this is extremely useful and greatly improves general XMPP user experience. I'm on my phone, read some messages from a group: I'd like gajim to show these messages as read when I get back to my computer. This is already working for direct messages, and it's great!

It may be reasonable to restrict chat markers to non anonymous, private MUCs, ie "groups".

FWIW, at least movim and Conversations implement this already (I think dino too).