Add a filter to hide translated strings

[oliver-phet]

This seems to be a simpler method to improve #302 and #337

It can be cumbersome for translators to identify missing (empty) strings. We'd like to either:

1. add a "filter" to Rosetta to hide translated strings (making it easier to find strings that are not yet translated).
2. Add a control to allow sorting the strings to show untranslated first

If we pursue (1) What happens when a user fills a string - does this disappear immediately after a translator enters something and moves focus to another element?

[oliver-phet]

Some quick ideas/mockups that we should discuss if/when this is prioritized: