Mail RID: (Parent-folder localid:IMAP UID)
Folder RID: (IMAP Folder path)

With this setup we don't have to adjust mail RID's if i.e. a folder is renamed/moved.
We also can treat the folder path's transparently, and can thus avoid the whole IMAP separator normalization.

It will require that the imap backend better supports actions such as createSubfolder, so we don't have to assemble paths in the resource.