* mail-format.c (get_cid): Make fake content-id URLs be guaranteed
	unique: the old way (with %p on the CamelMimePart *) would
	generate duplicates if memory was freed and re-allocated the right
	way.

	* mail-display.c (pixbuf_gen_idle, etc): Make the thumbnail cache
	global rather than per-MailDisplay, since content-ids ought to be
	globally unique. Also, don't leak content-id strings when the
	pixbuf generation fails, and remove pixbufs from the cache after 5
	minutes.

svn path=/trunk/; revision=8944