Rework build system to focus on central CMakeLists file

[dertseha]

This pull request reworks how to build the project. It is similar to #351 . It also allowed to build the thing again, locally.

Changes include:

• Removal of 5 nearly identical dependency-build scripts - dependencies can be integrated by CMake now
• Removal of redundant "32bit" CMakeLists file - Building a 32bit-binary is a cross-compilation topic
• Removal of make.exe - executable binaries in a repo don't feel like a good idea; also, redundant now
• Removal of 2 similar external CI services; Replaced with GitHub action
• Extraction of COMPILING.md, to better separate concerns. README.md is now more focusing on running it
  • COMPILING.md better elaborates how to do what

"Breaking" changes:

• CMake 3 is a must, and a rather high (albeit an already deprecated) version. Newest CMake 4 doesn't like the dependencies. This would be a greater follow-up project: GLEW should rather be replaced with GLAD, and FluidSynth-Lite seems not maintained.
• macOS build does not work. clang reports some alignment issues at linking time and I don't have the resources to investigate this. Either way, the Apple Silicon build is not verified anyway.