API, Slovak and performance improvements

New:

• The API provides two new methods to programmatically increase achievement category statistics. Thanks to @Tassu for his contribution!

Improvements:

• Small performance improvements to several achievements statistic trackers.

• The API Javadocs has been cleaned up and clarified in places.

Translations:

• The Slovak translation has been fully updated. Thanks to @KondzioSvk for his contribution!