Better visibility/control on synchronisation status

[julienmalik]

[enhancement request]

Implement tools to show/control the synchronisation status to the carddav server :

• "Sync now" button
• "Last successfull synchronisation on date:hour" message, next synchronisation in X minutes, error message of last synchronisation error...
• list of synchronized contacts (popup?), list of not-yet-synchronized contacts

During initial setup, it is currently hard to figure what goes wrong (adress of the carddav server, no synchronisation yet...). For everyday-use, we don't know when the synchro will happen. I can force synchro on my android phone (davdroid+baikal), but I can't on my PC through roundcube.