This (way too large patch) enables versioning in config files, and
provides a mechanism by which to load older versions from a newer
version of Geary. It also properly introduces a new v1 config format
that adds several groups to geary.ini to make it easier to read and to
distinguish between incoming/outgoig services rather than IMAP/SMTP.

To do this, a few things that should have happened in seperate patches
were also done:

 * Make AccountInformation's imap and smtp properties mutable (they
 aren't stateful any more anyway), make ServiceInformation non-abstract
 again and remove the subclasses (to get config versioning happening
 without an explosion of a classes, it all has to be handled from the
 AccountManager anyway), and some other misc things.