Rewrite TableView using pure javascript sets

[ci skip]
TableView doesn't use a QML Object as a set anymore, but uses pure
javascript sets now. Additionally, the height and width of top / left
headers do not have to specified manually anymore, as they are assumed
to be the same as the corners height / width.
Fix value-adjusting in ScrollBar

[ci skip]
The value adjusting that happens when the view's position is changed
from outside has been modified so that it works properly now.