Essentially we want code that works with QFile to also work for wasm.

The proposed solution uses an in memory file that is kept alive by QFile objects.

When the last QFile object disappears so does the file.

We will use the theme logic to delegate to our own file dialog. This means we

can no longer access the inmemory filesystem that emscripten provides.

Also, the filedialog can not return filenames as the file would be gone by

the time the user tries to use it. Instead it is necessary to use exec and attach

to the signal.