Make ImGui a git submodule of ImGui-SFML

[eliasdaler]

I think it'll be good to pin specific ImGui commit (I'll use release ones in "master" and releases) and allow to not specify IMGUI_DIR, but get ImGui from subrepository and be sure that the version you'll be using was tested with ImGui-SFML.

[RichardDally]

It brings more complexity for almost no gain, I prefer cloning imgui separately on my own.

[Alia5]

I guess you mean submodule?
Careful with the wording here 😅

In any case, I'd strongly argue for a submodule instead of git-subtree or subrepo and make sure that the path of ImGUI is configurable.
So don't take IMGUI_DIR away, but provide a default for a (may or may not) cloned submodule.

This way, folks who want to manually manage ImGUI can do so; For other folks, the submodule should be enough and works out of the box.