fixes #959: display DNA sequence on extension in order 5' to 3'

[dave-doty]

fixes #959: display DNA sequence on extension in order 5' to 3'.

This is done by always drawing the path for the extension 5' to 3'. Previously we drew it left-to-right, but in cases where the 3' end is to the left of the 5' end, this helped display the extension name/label right-side up, but caused the sequence to appear backward. Since the DNA sequence issue is more serious and could lead to mistakes in the design, for now we'll favor fixing that, and make separate issues to deal with

1. Displaying extension name/label right-side up in all cases.
2. Displaying DNA sequence right-side up. Currently, for cases where the 3' end is to the left of the 5' end, just like the default for reverse domains, the DNA sequence is displayed in the correct order but upside down. We'll need a separate issue to display it right-side-up when that option is selected in the View menu for reverse domains.