This kind of work will come after 0.13

This is the first commit of a long serious of cleanup commits.
Simple testcase included to test the slightly enhanced Interface dispatcher.

 - when a new contact with higher prio connect, detach session from chat control. Fixes #5021
 - don't re-use a session that was for another sessin

Tests are more than welcome!

The reworked version has refactored and simplified architecture, which also
involves simplified dispatching.

stop sending chatstates when remote server doesn't respond anymore. Don't acknoledge error message receipes. Fixes #4064

show only once unread messages at startup when we have twice the same contact in 2 accounts. Fixes #2921

[Dicson] add support for key 'body' in the Message Action, and add jid via roster command. Fixes #5195

 * add a checkbutton to use the entered nickname by default without asking anymore. Fixes #5141

restore metacontct correctly when we connect to server and we already have contacts in roster (stored locally). Fixes #5156

* In order to allow the viewing of the roster when we are offline (and some other cool stuffs), with and without roster versioning, now we load roster when RosterWindow is created (at gajim startup). Fixes #3190

[roidelapluie] Added initial support for Roster Item Exchange (XEP-0144) based on ticket #378. Only handling NS_ROSTERX 'set' operations is supported and tested as of now.

OS X support is still maintained in the osx_newbuildsys branch - and
only there.