option to display DNA right-side-up on all extensions

[dave-doty]

The quick fix to issue #959 was to draw all extension lines from 5' to 3' and link the textPath of the DNA sequence to that line. This means it displays correctly in order from 5' to 3', though upside down in some cases, see here: #959 (comment)

It would be nice, when the option View-->DNA-->Display reverse DNA right-side up is on, to display them right-side up, just like with domains. We'll need similar logic where it actually reverses the string so that the bases still appear in the correct 5'-3' order even when right-side up.