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.

[rezzeted]

You can't add the sources of another library to the sources of one library as a git submodule. This way you will link two libraries at the level of their source code. At first glance, this seems like a good idea, but think about how it will work in various package managers that build libraries from source code, such as vcpkg? After all, the package repository already contains imgui as a separate library, and which version of the source code should be used in this case will conflict.
The compatibility of versions should be thought about by the people maintaining the package manager repository, although this often leads to problems.