Message Inspector displayed always 0 Hz for Heartbeat

fixes segfault in local 3D view when starting QGC without having a UAS selected

cause: check for null pointer had no effect (static memory)
solution: fill mavlink messages (and thus) message type with 0xff. this patch assumes that message ID 0xff is never used
positiv side effect: qgroundcontrol uses 2% less cpu on my machine (eeepc)

Increased GPS sat timeout from 1 second to 10 seconds

sending GPS sat information via a low-bandwidth link like the UM96 @ 9600 baud should also work at low frequencies like 0.2 Hz

Various changes

Conflicts:
	qgroundcontrol.pri
	src/uas/UAS.cc

Changed the drop rate from a SYS_STATUS message to follow the MAVLink specs along with some extra range checking.

Added a workaround to the mission management code where it would only accept messages addressed to the MAV_ID_MISSIONPLANNER component. This was causing problems with the send-loss value being calculated, so I decided to add this until a proper solution can be developed.

Refactored part of MAVLinkProtocol::receiveBytes() as I was trying to debug within it. Nothing major, though I did remove an unnecessary loop.

Added multi-UAS support to QGCRGBDView. Fixed display timeout issue on computer for which data source is not time-synchronized.