Allow DropPanel dynamic sizing

Support for multiple links to a single vehicle

Appveyor only has Qt 5.9.5

FactValueSlider: Fix crash when empty

minor typo fix

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.

This setting enables/disables the LinkIndicator toolbar in which the primary link can be chosen.

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

Automated preflight checklists

Implement GeoFence persistence

Ignore old version 1

Automated preflight checklist: Initial commit. Note that this requires an autopilot firmware that writes the respective health flags in the SYS_STATUS mavlink message

Fix altitude change signalling

Vehicle. Expose sensor status to Qml

Simplify README with links to devguide

ArduPilot: Start possible Solo video