More accurate redraws

In this pull request:
* redrawing/marking areas as damaged is separated from actually flushing/doing these redraws ( #576 )
* Redrawing is made more accurate - e.g. only needed areas are marked as being damaged ( #349 )
* Move last remaining manual redraw calls into the aspect (was toggle) ( #486 )

I just ran through all the working samples and right now there are 2 problems persisting that I attribute to redrawing: 

[ ] simple-draw.rb "lags" e.g. lines only appear after some time/a line is kind of finished - either no flush is triggered or marking the damaged area happens to late
[ ] sample14 leaves artifacts of ovals behind --> some area is not marked as damaged

I also removed some specs speccing that `TextBlock#update_position` triggers a redraw. I moved the redraw triggering to one level above that, e.g. to `CommonMethod#_position` which is called by the Slot positioning code. This way it is more general and easier to define the methods (since they all rely on that method and I just have to add it to that module instead of every separate backend class, where I might miss some).

Happy for feedback of any kind =D