Remove unnecessary files from the Flatpak bundle

There are some files in the Flatpak bundle that we might not need to ship.

1. nw.js locales

Many files like this:

```
/files/lib/pd-l2ork/bin/nw/locales/am.pak
/files/lib/pd-l2ork/bin/nw/locales/am.pak.info
/files/lib/pd-l2ork/bin/nw/locales/ar.pak
/files/lib/pd-l2ork/bin/nw/locales/ar.pak.info
```

Each locale is about 700KB in total, so they are quite big combined. We don't want to remove localisation support though but maybe these can be split into separate per-locale packages?

2. Makefile and source code for extras modules.

There are many directories like this:

```
/files/lib/pd-l2ork/bob~/GNUmakefile.am
/files/lib/pd-l2ork/bob~/README.txt
/files/lib/pd-l2ork/bob~/bob~-help.pd
/files/lib/pd-l2ork/bob~/bob~.c
/files/lib/pd-l2ork/bob~/bob~.pd_linux
/files/lib/pd-l2ork/bob~/makefile
```

The -help.pd and .pd_linux file is needed, but the sourcecode, Makefile and README are not.
These all seem to come from the `./pd/extra` directory in the source tree.

3. `/files/lib/pd-l2ork/doc/media/midr-mjpeg.mov`

This is presumably example media for the documentation, but it's 3MB in size - check if it's really needed.