As preparation for more detailed definition of BorderLine
primitives I have adapted the BorderLine definition to
work with motre possibilities to define the LineStartEnd
definitions in a BorderLineExtend class. That one is
flexible to hold all kinds of definitions - from none to
all four possible extends (Start/End, Left/Right of vector)
Cleanup of DiagStyle and others: DiagStyle is not needed
anymore due to no longer using angles calculated, but
being based on vectors defining the geometry. Also cleaned
up quite a bit of no longer needed calculation stuff for
the control.