i18n: Add some comments to help translators

Our preferences dialog is unfortunately not as obvious as it should be.

The last thing we want is for translators to get it wrong and make it
even more confusing.

This should hopefully help.
i18n: Add some more translator comments

While we're at it, this is not entirely necessary, but it can't hurt.
i18n: And here's the new pot file
Merge pull request #25 from Cangjians/i18n

I18n