Update dependencies

[cschreib]

Changes in this PR:

• Remove submodules for dependencies, supplied CMake Find* modules, and pre-compiled dependencies. We now use Conan for dependency management. As a user for lxgui, you can choose to also use Conan (and you can see the GitHub action files for inspiration on how to do it), but you don't have to: you can manage your dependencies manually, or with other dependency management tools. Conan is only required for running the automated CI builds.
• Update dependencies for GitHub actions.
• Fix some CMake deprecation warnings.
• Fix some newly discovered compiler errors and warnings, from using more recent compilers.