Messages: add the prefix “Private” in MainWindow::msgTextMessage().
Messages: add the prefix “Private” in MainWindow::msgTextMessage().

Users can only detect private messages
if there is no prefix like “Channel” or “Tree”
in the messages.

This commit adds a prefix “Private” to the sender,
if MumbleProto::TextMessage contains a session value.

Fixes mumble-voip/mumble#2362

LuAPi/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble,LuAPi/mumble,Lartza/mumble,Lartza/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble,LuAPi/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble