Add a "parent" field to CamelFolderInfo.

Add a "parent" field to CamelFolderInfo.

	* camel-store.h: Add a "parent" field to CamelFolderInfo.

	* camel-store.c (camel_folder_info_build): Deal with "parent"
	(camel_store_folder_subscribed, camel_store_subscribe_folder,
	camel_store_unsubscribe_folder): Add g_return_if_fails checking
	that the folder supports subscriptions.

	* providers/imap/camel-imap-store.c (folder_subscribed,
	subscribe_folder, unsubscribe_folder): Remove "+ 1"s since the
	mail subscribe UI won't prepend / to the folder names now.
	(get_folder_info): Clear the "parent" field of the folderinfos
	when removing an empty top level.

	* providers/nntp/camel-nntp-store.c (nntp_store_folder_subscribed,
	nntp_store_subscribe_folder, nntp_store_unsubscribe_folder):
	Remove "+ 1"s since the mail subscribe UI won't prepend / to the
	folder names now.