This PR contains several improvements to the build system:
Semi-manual Docker-based process is replaced with a fully automatic one using GitHub actions. Instead of using containers and cross-compiling, CMake scripts are now designed to build natively and we use GH Actions Linux, Windows and OSX workers to build the respective versions of the plugin, all of which then get bundled into the plugin archive. Build-related documentation is updated accordingly and placed in the README for better discoverability.
*.xpl binaries are no longer committed into the repo, since GH builds them automatically for every push.
Docker scripts are now removed, but if at some point we need them back, git history has them.
I purged a whole bunch of unused/unnecessary binary blobs and headers from the repository to reduce confusion and chances for the wrong library getting linked in. There are still things that could be improved on that front, but at least all remaining blobs are in use (or I think they are).
This PR contains several improvements to the build system: