tests: Reset sys.path to its original value

We only need to add the current directory temporarily to sys.path, so we
can hack Python's import machinery into loading the source tree rather
than the installed module.

Once that's done, we really should get it back to normal, to avoid any
unwanted side-effect.
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?
Merge pull request #42 from Cangjians/prefs

Prefs