Use po/LINGUAS

In most official Gnome projects the `ALL_LINGUAS` macro in `configure.ac` has been replaced with a file called `po/LINGUAS` (intltool >= 0.35 is required). This way translators just have to care about the files in `po/` and don't have to edit `configure.ac`.

As this is a relatively small project it might not be as useful as in other, bigger projects. However, it might be a good idea to be consistent with other projects.

For further details: [«GNOME Goal: LINGUAS»](https://wiki.gnome.org/Initiatives/GnomeGoals/PoLinguas)