tests: Ensure the preferences UI file is valid

This will ensure that the file is valid XML, which is a useful first
level check.

Obviously, this fails... for now!
tests: Ensure the preferences UI file is valid

This will ensure that in addition to being valid XML, it is valid for
Gtk Builder.

Obviously, this also fails... for now!
prefs: Fix the ui file

Fixes #33
i18n: And change those files again

This is annoying, is it even necessary?