App/Playtime: Add Linux support

[helgoboss]

This will probably happen in 2 stages:

1. Make the app available as standalone app (not embedded into REAPER). This is not too much effort and would already make it usable, but with a few annoying UX issues.
2. Embed the app into REAPER (as done on Windows and macOS).

[mikobuntu]

Just checking in to see if there has been any progress on this ? Thanks :)

[helgoboss]

No, busy with fixing bugs / working towards the final release (my first priority right now).

[mikobuntu]

OK thanks for the update, much appreciated :)

[helgoboss]

Guys, I'm starting to work on Linux support stage 1. You can follow the progress here: #1391

[helgoboss]

Linux support stage 1 is here!