XEP-0313 (MAM) Implementation to fix #10

As updated in issue - should be ready for review. I don't like the approach myself (to collect message by pieces) but unfortunately libpuprle does not expose jabber parser.

re GUI - need some way to either have way to insert properly ordered messages (by timestamp, or introduce sequence) or to re-load message-list after history is updated by mam.

Also needs some GUI for prefs. MAM preference should potentially be per-account but also with global toggle (total disable) perhaps. Currently it checks per-account. Need to add per-room as well actually.