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.

This ensures no ports are being displayed to the user that are definitely unusable.

We were connecting to system peripherals before, which adds boot delay as QGC re-tries opening the port. We were also opening serial ports two times on Mac OS: Once as /dev/cu.usbmodem1 and once as /dev/tty.usbmodem1 - since the cu.usbmodem alias was already open, the 2nd opening always failed, leading to further boot delay.

Instead of hardcoded "QGroundContorl" strings

Prevent shutdown ordering crashes

No need to add dynamic MockLink comm links

No signing support yet

_finalize methods need information which is not available if all you
have is a mavlink_message_t

Also change Plan Element selection ui

* Add support for another board

* Added a 2nd custom board