Add a filter for "New" and "Changed" strings to Rosetta

This would also require modifying the UI for 'PO file downloads' to also include a boolean like "Set as last synchronisation date" for Project admins as well.

Though, I don't think these filters belong in the translate form (we'll have there filters like "With new suggestions" and "Changes in Launchpad"), but rather on the template overview page.

Contrary to what you say, the "New in Launchpad" should also be in the translate form.

For package maintainer there should be an easy indicator on the "template overview page" telling whether a language changed (new, changed strings) or not. The colored bar is nice but works only for large numbers. I would suggest to add a small mark behind that colored bar (e.g. a star). This is used in many text editors to mark changes in window title and should be easy understandable (when documenting it on mouseover).

IMHO, ideally, "New in Launchpad" would be included in those listed among "Changed in Launchpad". It's currently not so because "Changed in Launchpad" was designed to help with reverting translations to "packaged". I believe we should modify this to include those translations that were newly provided in Launchpad as well (i.e. they are both useful for seeing what has recently changed and to submit only these changes upstream).

That way, by using "changed in Launchpad", maintainers would be able to see everything that has changed since their last sync (atm, sync happens when maintainers do a tarball upload of translations, and we might want to keep using that as the sync mechanism, along with branch sync and whatever else we introduce in the future).