Options UI cleanups

[matatk]

Now that there are several options relating to the border drawn around landmarks, it makes sense to clear up the UI a bit. This may be left as-is until headings are supported too, but when they are, the following probably make sense:

• Instead of having momentary/persistent/none as border showing settings, there should be an overall "Show landmark (or heading) borders?" option for each type of border.
• Having each group of options under a <fieldset>.
• Putting advanced options (currently only the one, debugging) under a disclosure widget.
• Adding help info directly into the options page.

Should the options page be made more visible? I have resisted this so far, as there are already mechanisms provided by the browser to access extensions' options.

[matatk]

Layout considerations: separate panels for landmarks and headings, or a more table-like approach?

[matatk]

It could be that some users might want to use the pop-up for landmarks and the sidebar for headings, so it should be possible to support that.