Add camel_imapx_folder_path_to_mailbox().

Converts a folder path to an IMAP mailbox name, which just replaces all
slashes with the mailbox separator character.  If the mailbox separator
character appears in the folder path, it is replaced with a slash.
imapx_store_rename_folder_info: More cleanups.

CamelIMAPXStoreInfo now holds the mailbox separator character alongside
the mailbox name, so we don't need to consult namespaces to convert the
folder path to a mailbox name in imapx_store_rename_folder_info().  Use
camel_imapx_folder_path_to_mailbox() along with the CamelIMAPXStoreInfo
separator character.