Remove exists, create, delete. A CamelFolder now always references an

Remove exists, create, delete. A CamelFolder now always references an

	* camel-folder.c: Remove exists, create, delete. A CamelFolder
	now always references an existing folder. Remove delete_messages
	too since it wasn't being used. Add a "create" flag to
	get_subfolder saying whether or not to create the subfolder if it
	doesn't yet exist.

	* camel-store.c (camel_store_get_folder): Add a "create" flag to
	say whether or not to create the folder if it doesn't yet exist.
	(camel_store_delete_folder): New method, moved from CamelFolder.
	(cache_folder, uncache_folder): Fix up a bit.
	(get_folder_name): Explain what this is for.

	* providers/mbox/camel-mbox-folder.c:
	* providers/mbox/camel-mbox-store.c: Update. Remove support for
	hierarchical folders to simplify this for now, since we're not
	using it, and it's not completely clear how they should work in an
	ELocalStorage world. Needs to be revisited.

	* providers/pop3/camel-pop3-folder.c (delete_messages): Remove.
	* providers/pop3/camel-pop3-store.c (get_folder): Update.

	* providers/vee/camel-vee-folder.c (exists): Remove.
	* providers/vee/camel-vee-store.c (vee_get_folder): Update.