[CalDAV] Force password ask on soup authenticate failure

Some servers may allow OPTIONS request on authenticated account with
a wrong password, but fail on REPORT and other queries, which makes
the calendar open routine not recognize the wrong password case.
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.
IMAPX: Remove some unused character macros.
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.
Assamese translation updated