libhelium Kotlin DSL

[lleyton]

Essentially a DSL for building libhelium apps in a reactive and declarative fashion. We hope to significantly ease the developer experience especially for with little native/GTK experience, hopefully providing the easiest way to build beautiful native desktop applications on Linux and other platforms.

gtk-kt (now dead) has quite a bit of work on a Gtk DSL, which we could port over to gtk-kn. Depends on #25.