Initial support for themes + Linux, MacOS and Windows build support.

IMGUI is a code-driven, simple and bloat-free GUI system, widely used in modern game engines and games. Best explanation of IMGUI concept is here. The library has basic suppot for layouting (attachable toolbars, layout serialization), themes (colors and serialization), lock-free multithreading (triple buffering).

This library was a part of my game engine, but I extracted and moved it to github some time ago.