It is just a simple general-purpose, data-oriented, data-driven, entity-component based, lua scripted game engine written in sane C++ for my own game development needs.
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.
I do not use STL because I want consistent implementation across all supported platforms.
Also, you have to care about memory management when you need performance or when working with memory constrained devices. The way STL deals with custom allocators makes no sense to me, hence my own implementation.