Pull latest translations from Transifex for 2.2.x.

[rryan]

See commit messages for the precise command run at each step.

I also re-built mixxx.ts but there have not been any string changes, so this is a no-op.

Based on discussion in #1610 and #1796, I used 1 percent as the minimum threshold, which brought new languages:

• de-DE
• en
• es-MX
• fr-FR
• zh-TW.Big5
  Do we want these?

[rryan]

The first two commits are to fix warnings produced by lupdate.

[rryan]

Since we're publishing a release candidate on Nov 3, I plan to merge this beforehand.

One final ping to @esbrandt and @sblaisot -- we can always revert these commits later.