camel_imapx_mailbox_is_inbox: Minor cleanup.

Rename 'mailbox' parameter to 'mailbox_name'.
Add camel_imapx_mailbox_to_folder_path().

Replaces camel_imapx_store_summary_mailbox_to_path().

Converts the given mailbox name to a Camel folder path, which just
replaces all hierarchy separator characters with '/'.  If '/' appears
in the mailbox name, it is replaced with the hierarchy separator.