Patch: add a reason to tny_camel_folder_get_headers_async operation

[José Dapena Paz <jdapena igalia com>]

	Hi,

	This patch adds some reasons to get_headers_async operation, as it
could happen that, while the operation was running for renewing the gtk
list store model headers, we got 0 reasons (all old headers removed but
no new header added). Then we were freeing the message cache.

	This caused some not so infrequent crashes.

	The changelog entry is this one:

* libtinymail-camel/tny-camel-folder.c:
        * Add some reasons/unreasons in get_headers_operation, to avoid
          getting a 0 account when we are replacing the headers in the
          gtk list store model. This should avoid some very hidden
          crashes.

-- 
José Dapena Paz <jdapena igalia com>
Igalia