desktop: check that no remaining TODOs are in appdata file.
Before each release, we prepare now a <release> tag. Unfortunately some details are not always well known for sure in advance, in particular the date of release (plans are made to be broken!), and also the release news URL (since the date is in the URL). So I usually leave them as TODO and until now, I never forgot to update them just before release. But there is always the risk of forgetting. Now the test for the appdata files will also look for remaining "TODO" in the file if and only if the micro version is even (which means it's a release). So we won't ever forget to update the file as long as we run `make check` on the last commit (which we always do obviously).