Server-side memory leaks in calendar factory

This change addresses:
- typo for GDestroyNotify (was being used ref, instead of unref)
- leaking data_cal object when opening a calendar
- DataCalView dispose didn't remove view from the backend

These led to memory leaks on both view and backend objects.
[CamelGPGContext] Prefer gpg2 over gpg

If there is a newer version installed, then use it, rather than
the old.
Bug #702709 - Do network background operations only if the CamelSession is online

This may prevent any network background operations, when associated
CamelSession is offline, like when the machine gets suspended or is
just resumed.
Bug 722275 - IMAPX: Parser trips on mailbox containing '+'

Mailbox names containing a '+' character were tripping up the parser due
to it being treated as one of a special set of characters to distinguish
identifiers from IMAP syntax.  But I don't think '+' is necessary in the
set of characters.  For continuations, at least, it should always follow
a newline character, with is also a special character.

(cherry picked from commit 34f0f99f715bb3e308f1d838fac435b3d5fb6925)
Assamese translation updated