Show only mail messages in mail folders

Some folders can contain not only mail messages, but also pure
calendar/contacts/... items, which the mail (Camel) folders do
not operate with well, but more importantly which do not belong
there, thus skip those items in mail folders.

This is related to the evolution-list mailing list thread:
https://mail.gnome.org/archives/evolution-list/2015-September/msg00013.html