Improve build system

[pixelomer]

The current build system uses several Makefiles that are designed to only work in very specific conditions:

• For Linux: No specific conditions
• For Windows: Fedora Rawhide with Qt6 Linux and MinGW development packages, MinGW x86_64 and i686 tools (gcc, g++, etc.). See dev-docker/. The Makefiles look for MinGW tools at non-portable locations. Ubuntu is not supported. Older Fedora versions produce unusable binaries.
• For macOS: Qt6 installed with MacPorts. Qt6 install location is hardcoded in the Makefiles.

Ideally, it should be possible to build Shijima-Qt in other configurations as well. For example, these configurations are currently unsupported for building:

• Building for macOS with Qt6 development packages installed with Homebrew
• Building for Windows on Windows, macOS or Ubuntu
• Building fat macOS binaries or arm64 Windows binaries

Switching to CMake for the build system would be a good first step.