Add feature to only add new strings from an en.catkeys file

[humdingerb]

Ease updating of already translated catkeys with a newer en.catkeys file. For example:

Add a menu with an item "Update from..." that opens a file panel for the user to choose an updated en.catkeys file. Compare the original strings in the new en.catkeys file with the ones in the old translated catkeys file. Add the newly added files to the translated catkeys, as well as the new fingerprint ID in the first line of the file.

Now unused strings in the translated catkeys should be marked as such, not simply removed. The original string might only have changed a little and the already translated contents could be reused.
When closing the CatKeysEditor, the user should be asked if the unused strings should be removed.

[humdingerb]

This was partly implemented by Brian with b4b4e96.
It would still be nice to keep the already translated, now out-of-date, phrases around. Maybe like described above, or something better...