[cmake] use ImGui with CMake

[brccabral]

Hi,
I have managed to setup some cmake config files to ImGui.
It detects the dependencies and setup the backend targets accordingly.
I just could not test the Apple examples, but it should not be difficult.
The examples became only a few lines, the heavy lifting is in the backends.

Users have two options

1. They can compile ImGui and install in the system, then in their projects just call find_package(imgui), or
2. They can copy to a subfolder like third-party/imgui and just call add_subdirectory(third-party/imgui)

Let me know if you have any questions.

[mars3142]

I could test the macOS part, if needed.

Did you test, if ImGui is compiled from source within the project? This is my normal setup, because I always use a well defined version with FetchContent_Declare. So I can download a specific tag for my projects and don't need to install it at first on my build/dev server.