You definitely have to use switch-control to receive commands:
Thing device 5094TSM_living "Push button living" @ "KNX"
{
Type switch-control : Channel_84 "Projector aan/uit" [ ga="1.001:3/4/0" ]
}
maybe don’t set the DPT…
Type switch-control : Channel_84 "Projector aan/uit" [ ga="3/4/0" ]
Of course, 3/4/0 has to be the GA which is sent from the wall switch 
If you want to send a status to the wall switch, be aware that the status GA has to be the first GA:
Type switch-control : Channel_84 "Projector aan/uit" [ ga="3/4/1+3/4/0" ]
So, openHAB will send the state to 3/4/1 but will receive commands from 3/4/0.