WASM: unable to set moduleConfig

When building for the webassembly target, you may want to configure the emscripten module (https://emscripten.org/docs/api_reference/module.html) by setting cli arguments, or override the default behaviour of print/printErr.

The QtLoader class encapsulates moduleConfig in a way that doesn't allow me to provide these without patching.

Additionally I'm missing a signal that tells me that the module is ready to be used on the JS side. In earlier versions there was a global Module object which was already initialized and usable when showCanvas was triggered. That isn't the case now, and there's no alternative way.