This makes translation easier as there's a single file to edit at.
Existing short and full descriptions are conserved.
sync_translations.py takes care of updating the metadata files.
The metadata directories are renamed to match the language codes used in `res/`.
Contributing guidelines are updated accordingly.
`gen_layouts.py` lists the layouts in `res/xml` and generate the
`pref_layout_values` and `pref_layout_entries` arrays into
`res/values/layouts.xml`.
These arrays are hard to maintain as the order has to match, which is
fragile.
This relies on every layouts having a `name` attribute.
The new script makes sure that strings files don't contain obsolete
strings but also ease the job of contributors by adding missing
translations as comments.
A Github Action ensures that translations stay in sync over time.
