Set the disconnected status. (camel_disco_store_can_work_offline): Return

Set the disconnected status. (camel_disco_store_can_work_offline): Return

	* camel-disco-store.c (camel_disco_set_status): Set the
	disconnected status.
	(camel_disco_store_can_work_offline): Return whether or not a
	given CamelDiscoStore can work offline or not.

	* camel-disco-folder.c (camel_disco_folder_cache_message):
	Explicitly tell a folder to cache a message. (Better than using
	get_message, because for IMAP that doesn't guarantee you'll get
	all the message parts.)
	(camel_disco_folder_prepare_for_offline): Prepare a folder for
	offline use by caching all messages meeting given search criteria
	(and doing anything else the particular folder implementation
	needs).

	* camel-session.c (camel_session_set_online,
	camel_session_is_online): A session-wide online/offline toggle.
	(camel_session_init): Set online to TRUE.

	* providers/imap/camel-imap-store.c (can_work_offline):
	Implementation of CamelDiscoStore::can_work_offline. (Checks that
	the store has been used online at least once.)
	(imap_get_folder_online, imap_get_folder_offline): Deal with
	request for "inbox" properly. ("Don't you mean... 'INBOX'?").

	* providers/imap/camel-imap-folder.c (imap_cache_message):
	Implementation of CamelDiscoFolder::cache_message.

	* camel.h: Add camel-disco-store.h and camel-disco-folder.h