'Adds the specified flags to the message\'s flag set.
If a given flag is already present, it will not be duplicated.
Note that this function does not check against dryrun settings,
so you need to ensure that it is never called in a
dryrun mode.
:param flags: A set() of flags'

def addmessageflags(self, uid, flags):

    newflags = (self.getmessageflags(uid) | flags)
    self.savemessageflags(uid, newflags)
