Refactor all Singletons

I've decided, that all Singletons (but ProjectManager) are/still remain accessible through getInstance() because there shouldn't be performance issues in the IDE.

ProjectManager still has a public static final instance variable to double-increase the performance during the stage / actions execution. (1. its a member and not a method and 2. it's final)

The jenkins testrun is running...
Atm, I will add some comments to explain my decissions in more detail.