New code to keep an on-disk cache of what UIDs have been seen in a folder.

New code to keep an on-disk cache of what UIDs have been seen in a folder.

	* camel-uid-cache.c: New code to keep an on-disk cache of what
	UIDs have been seen in a folder.

	* camel-provider.h: Add new flags CAMEL_PROVIDER_IS_SOURCE (mail
	can arrive in it by non-Camel means) and CAMEL_PROVIDER_IS_STORAGE
	(you can work with mail directly without needing to copy it local).

	* providers/*/camel-*-provider.c: Add flags as needed: imap and
	mbox are SOURCE and STORAGE. mh and nntp are just STORAGE, pop3 is
	just SOURCE.

	* camel-mime-message.c (process_header): Add another subject
	g_strstrip that fejj's earlier commit missed.