move styling from javascript to css in RNAplot and fornac

[j-4]

RNAplot and fornac currently define many colors and other styling options within the javascript code. It would be much more universal, if we would move this information to the css styles.

E.g. color schemes could work like this:

<g class='rna basepairscheme'>
  ...
  <circle base_type='a' domain_type='r'>
    ...

and css:

.basepairscheme > circle.node[base_type="a"] {
color: red;
}
.domainscheme > circle.node[domain_type="r"] {
color: blue;
}

By exchanging the basepairscheme class to domainscheme class, we could color the circles efficiently without any javascript code. Customization of the colors could then be done in css.
Exceptions for this approach would be coloring by position or custom coloring.

[j-4]

This will also fix ViennaRNA/forna#32