Unread messages include all MUC messages

Bug description

gajim-remote get_unread_msgs_number includes the count of all unseen MUC messages, not just ones mentioning me. This prevents me from using this number to power any useful visual indicator.

Steps to reproduce

Join a MUC
Minimize the MUC
Wait until a message is received in (join elsewhere from a different JID and send a message if you are impatient)
Run gajim-remote get_unread_msgs_number and notice the nonzero count

Software versions

OS version: Debian jessie