Improve loading session possibilities

A LoadingSession is some parameters, defining how emails should be loaded, and from which sources.
Current parameters are mainly:

• max number of emails to load in a session per server (-1 = all)
• list of servers (defaults to all)
• list of stores (defaults to none)
• if should read only emails without the "read" flag (defaults to true)
• if should focus on "recent" emails (defaults to false, quite blurry feature imo)