Draw lines between pointers and related columns
Introducing a new class that draws lines between the pointers
and the related diff columns. Lines are redrawen when pointers
were moved. A colured border is added to the diff columns and
readded when the diff changes.
Other then in the mock, when a pointer moves to the side it is
not hidden and the line is not ending in the arrow-bar. This
is done because it would hinder the user to drag the pointer.
Since "dragging instead of clicking" is the desired way to go
with the extension this should be avoided.