Resource system backend for Qt on WebAssembly

It does not make much sense to pipe (large) resource sets though the Wasm compilers.

Instead, resource files can be copied to the in-memory file system provided by emscripten, where they can be accessed using standard file API. The Qt resource system will then map qrc paths to filesystem paths, e.g. ":/foo" -> "/qt_resources/foo".

Build flow:

1. QRC copies resources to a qt_resources/ dir (on the host/build machine), instead of creating qrc_.o files
2. The emscripten file packing feature is used to populate the filesystem: "em++ --preload-file qt_resources/"