2.3.0-beta.1

Rough initial support for mixed 1st generation and 2nd generation UltraFine setups.
If no built-in display is found, the app will now use the first 2nd gen UltraFine it finds as a source. (#17)
It will no longer try to sync to a 2nd gen UltraFine (as they support auto brightness).

This for now only takes into account the 5k UltraFine model, because I use the model number to distinguish and I don't know 4k UltraFine model numbers.
If you have a 2nd gen 4k UltraFine, you could help by providing the model number Brightness Sync reports by opening the menu in the menu bar and pressing ⌘C to copy some diagnostics about your connected displays to your clipboard (the menu will close).