Wow it seems to contain a full level editor written in vala, a language I hadn't heard of. It seems to be a high level language similar to C# but with a native compiler and some different semantics (RC instead of a tracing GC for example). Is vala widely used?
Vala came out of Gnome/Gtk app development on Linux IIRC. It takes the GObject object layer from Gtk/Glib and promotes it to a first class part of the language: There’s a fairly direct translation from the various parts of the Vala language to C + GObject / Glib / Gtk+.
I don’t think it’s widely used outside of the Gnome desktop world, but there’s quite a few apps written in it out there.
Toolchain was initially written in C#. Vala simplified a lot the switch to GTK+3.
Today I think an IMGUI-like approach is the way to go for game tools. Rewriting the editors should be rather painless due to their engine-decoupled TCP/IP architecture.
Vala has been out for some time. I haven't used it since 2007 though. Vala would always come up in discussions about C# on GNU/Linux and Stallman in particular wanted no parts of C# or Mono in the operating system.
