change the CamelFolderSummary interfaces to allow partial summary queries

change the CamelFolderSummary interfaces to allow partial summary queries

	* camel-folder-summary.[ch]: change the CamelFolderSummary
	interfaces to allow partial summary queries (for dealing
	with very large folders). Remove the "extended_fields" from
	CamelFolderInfo and CamelMessageInfo: this is better dealt
	with by subtyping.

	* providers/mbox/camel-mbox-summary.[ch]: Make CamelMboxSummary a
	subclass of CamelFolderSummary. Update interfaces for that. Remove
	the internal/external summary distinction. Remove the (unused) md5
	checksum in the folder summary. Change the summary file format
	(primarily to make it no longer byte-order dependent) and add a
	version number to it so it will be easier to change in the future.

	* providers/mbox/camel-mbox-folder.[ch]
	* providers/mbox/camel-mbox-search.c
	* providers/mbox/camel-mbox-utils.c: update for summary changes

	* camel-exception-list.def: add
	CAMEL_EXCEPTION_FOLDER_SUMMARY_INVALID