DroneCAN UI issues

[tridge]

Many thanks for adding multicast support to the DroneCAN UI!

Could you make it possible to switch between CAN1 and CAN2 with MAVCAN? You just need to change which bus you send in the CAN_FORWARD request.
Or make the "Connect" button become disconnect when connected? Then users could disconnect and re-connect to the 2nd bus.

I also notice that MissionPlanner is sending an odd sequence of register/de-register requests for CAN forwarding when using MAVCAN. It does seem to settle down after a few seconds, but we do get multiple register/de-register requests