Support for running Avitab as standalone add-on to Microsoft Flight Simulator 2020

[mjh65]

This update supports the build of a new Avitab standalone variant that can be run alongside Microsoft Flight Simulator 2020. There are currently many restrictions, but it may be useful for some small subset of VR flyers. The new executable will only be created if the MSFS SDK is installed on the build machine. Env. var. MSFS_SDK msut be set. The new executable [Avitab-msfs.exe] still uses the X-Plane installation to read NAV data, so it's only really useful for VR flyers who have both sims installed. The Map app will centre on the user aircraft current location. Any other aircraft that are shown will be simulated AI aircraft, not other simulators in a private group, or in the worldwide public gamesphere. This is a known and reported defect of the MSFS SDK. When used in VR the Avitab-msfs window will need to be 'imported' into the VR space, eg using Steam's desktop window overlay support, and some jiggery-pokery will be necessary to switch focus between MSFS and Avitab to target mouse/VR controllers as required. In summary, this is a very early proof-of-concept update, but perhaps still worth wrapping into the main Avitab repo?