draw link lines with a width

draw link lines with a width
ticket:134
Segments connecting two sequence locations together now have a "length"
field which is drawn as a width across the screen.

The problem with this patch is that on the sequence view, where you want
a narrow line, it's drawing something the width of the base pair.

perhaps that's ok, or perhaps in that case I can have a length of 0,
or perhaps lenghts of 1 could also be drawn as a line, or perhaps
the edges of one of these rectangles should be shrunk a bit.

(that might also make things look a bit more centered.