Add translation system for .desktop and source file

This adds a translation system to the Makefile. It also adds a .pot file with all the current translatable strings and a (poor) start for the German translation (mainly for testing purposes).

You can add .po files easily via "msginit -i po/gtk-theme-config.pot --locale=de", then move the generated de.po file to the po/ directory, edit it accordingly and add it to git to make everything work. The make setup will pick it up automatically when "make install"ing.

You may want to consider switching to a more sophisticated build system like autotools or cmake, but this works too.