Moved the impl to the src/ folder so I can start working on a proper PIXI ImGui implementation (probably with render-to-texture)

Background color, start looking at better PIXI integration (need to be pixi-aware in imgui_impl)

ImGui 'kind of' works in the PIXI renderer, but it seems scaled (DPI aware now???) and the mouse doesn't work

Fixed the imgui project so we can do a dev (incremental) build and a distribution build

Start prepping for dev SystemJS build