Add support for specifying fixed RTK base position

Fix critical check

PX4 prearm checks severity

Change to case insensitive compare

Move ArduPilot preflight check handling to generic QGC

Fixed the core MAVLink receiver. Among other things, it wasn't accounting for fragmented messages.
Fixed code that monitors messages received (and lost)
Added a section within the MAVLink panel to show the current status (for the current vehicle)
Remove some old, unused exposed interfaces

Also fixing some c++ errors within commented off code as the localization engine was getting confused by it.

Fix negative wind direction

Show the link indicator in the tool bar if two or more links are connected and at least one of them is a high latency link.
Show in the link indicator if a link is inactive.

- Improve function names
- For notifications of an active link change only show the priority keyword if multiple links are connected
- Use QMap instead of QList for the HeartbeatTimers
- Style improvements
- Simplify sending the MAV_CMD_CONTROL_HIGH_LATENCY command

Add the HeartbeatTimer class to track a single heartbeat for a single link.
For every new received heartbeat an instance of the class is created.
Every instance emits a signal if it timed out or the timer is restarted.
Each vehicle then decides based on all different signals which link is the
priority link.

Each link updates the active property based on incoming bytes.
The vehicle tracks the active property of each link and in case of an inactive priority link it updates the priority link.

The LinkIndicator shows the connected links and lets the user select the priority link,