Remove unnecessary files from the Flatpak bundle

[ssssam]

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.

[ssssam]

Done in #8:

1. Include files:

/files/include/pd-l2ork
/files/include/pd-l2ork/g_all_guis.h
/files/include/pd-l2ork/g_canvas.h
/files/include/pd-l2ork/m_imp.h
/files/include/pd-l2ork/m_pd.h
/files/include/pd-l2ork/s_stuff.h
/files/include/pd-l2ork/s_version.h

2. /files/lib/pd-l2ork/Makefile.am and /files/lib/pd-l2ork/README.txt

3. /files/lib/pd-l2ork/bin/nw/credits.html - a 4MB file that ships with nw.js that seems to contain license text for every module that nw.js contains. We can remove.