wasm: Possibility to override default .html and .js files

WebAssembly builds copy wasm_shell.html, qtloader.js and qtlogo.svg to build directory. There is no way to override these.

This is probably fine for testing usually, but in some more complex applications there could be interaction with the page itself, which could require customized html file. The file could be modified or replaced locally, but that would have to be done by every developer and every time Qt kit changes.

For Android there is ANDROID_PACKAGE_SOURCE_DIR variable which kind of solves similar problem. Perhaps something like that could be added for WASM (contents of the directory copied to build directory).