Only update SEEN flag when it is the only allowed

If the server says in the select command that a folder
is read-write Kontact assumes that it can modify a basic
set of flags. (See the bug for more info) But if we can
only modify SEEN we can reuse the setSeenFlags functions.

This is a bit of a workaround for a proper and generic handling
of the permanentFlags but since this is only relevant for
the old code I did not want to change more then necessary to
solve the concrete Problem.

Fixes:
kolab/issue4868