I've decided to open source Ebou, the cross platform Mastodon Desktop Client.
You can find the repository here:
https://github.com/terhechte/Ebou
It also supports Windows, although this is beta and Windows binaries are not included yet (you'll have to compile it yourself). Attached is a Windows screenshot. Linux should be easy to support, too.
I'm open sourcing it because I think there's great value in a high quality cross platform Mastodon desktop client and I can't pull this off alone.