Allow to reset to factory defaults #11
Description
There was an issue on AvNav wellenvogel/avnav#161.
It turned out that the AvNav config was completely corrupted.
AvNav now has some internal handling to potentially fall back to a factory default config if it's config is corrupted and there is no "last known good" config.
For OpenPlotter this could become an issue if the user changed the AvNav HTTP port as AvNav would fall back to it's default port - and maybe could not start due to port conflicts.
On OP it could make more sense to give the user a chance to reset the AvNav config by hand (for example if AvNav is not starting any more).
By triggering the OP port check at this point the user could be given a warning on conflicts and an option to change the port settings.