imgui docking 1.90.1 imgui_glfw_opengl2 multi platform compiler configuration

the CMakelists.txt works on local builds for Linux, MacOS, and Windows. The Windows build works via GitHub actions. The others do not.

MacOS CMakelists.txt reference: https://gist.github.com/nebgnahz/b10dc964b7541f125696a2ea79350333 (completely useless don't use this)

MacOS CMakelists.txt reference: https://github.com/morizotter/imgui-opengl-glfw-glew-cmake-demo (helped somewhat)

Original unmodified source code: https://github.com/ocornut/imgui/tree/docking

File dialog reference implementation: https://github.com/aiekick/ImGuiFileDialog