Add methods to grab the current screen content as Buffer

**Short overview**
Similar to `Screen#capture` and `Screen#captureRegion` functionality to grab the whole screen content or a specified region would improve flexibility.
This could also be seen as preparation to support buffers in `Screen#find` one day.

**Detailed feature description**
Provide methods `Screen#grab` and `Screen#grabRegion` which return screen content as `Image`