Remove redundant desktop file entries on installation

Many desktop files ship with redundant localized entries (read: same value as default). This is highly annoying, and can lead to annoyances like https://github.com/TheAssassin/AppImageLauncher/issues/62.

As I recently learned, some translation tools that natively support desktop files (like Transifex) are to be blamed for this situation. Once a language is added to the project, it seems all entries are localized. There is apparently no way to prevent it from doing so.

In order to prevent such situations from happening again, I thought it might be a good idea to clean up those redundant entries. We can use the code from #820 to get a list of languages for every entry type, compare them to the default value and remove the unnecessary entries. That'll also compact the desktop files, making them easier to read.