Kill JumpKeyHandler and replace it by proper action shortcuts

Kill JumpKeyHandler and replace it by proper action shortcuts

The so-called "RTS Shortcuts" (now called "Quick Access") were until now implemented through
JumpKeyHandler, which installed event filters in appropriate widgets. Instead, we can just as
well use normal actions and simplify the code (which showed its age anyway).

As a bonus, the Quick Access shortcuts are now configurable!