ENVELOPE support in trunk

[Sergio Villar Senin <svillar igalia com>]

Hi,

I have just committed in trunk the code that retrieves
ENVELOPE+BODYSTRUCTURE instead of HEADERS for IMAP in tinymail trunk.
The speed up is really huge with some servers. For example in a rough
test I run a couple of hours ago, downloading ~1500 headers from AOL
server lasted ~5 minutes more when retrieving HEADERS.

The con is that now we always write *all* the BODYSTRUCTUREs to the
cache. This increases a bit the size of the cache but the good thing is
that it will enable us to easily store them in tracker for example.

Another con is that now we don't retrieve the X-MSMAIL-PRIORITY priority
header. I think it's a good deal anyway. BTW, we don't retrieve the
X-MS-HAS-ATTACHMS header either, but no problem there as I added a
heuristic that provides the same information by examining the BODYSTRUCTURE.

Br