imapx_store_rename_folder_info: More cleanups.
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.

matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server