WebAssembly: Support multithreading for shared libraries and dynamic linking

Enabling threads currently results in a non-functional build:

• Emscripten does not call locateFile() when downloading shared libraries from workers, which means that we can’t redirect the download to “qt/lib”
  Workaround: copy or symlink Qt libraries to application directory.

• Emscripten downloads and instantiates each shared library for each web worker, which causues the browser to run out of memory.
  Workaround: build in release mode / with optimizations

• Finally, there is an error on startup:
  Uncaught (in promise) LinkError: imported shared memory but unshared required