Add separate ThumbnailURL function

Common.m now has PreviewURL function for generating preview HTML, and ThumbnailURL function for generating thumbnail SVG.
Use WebView to render SVG thumbnail

Open Babel is used to generate an SVG representation from the file contents, which is rendered in a WebView, which is drawn onto an NSGraphicsContext.
Change UTTypeConformsTo to public.plain-text

OS X makes some decisions about how thumbnails are displayed using the file UTI. By stating that these formats conform to `public.plain-text` instead of just `public.text`, OS X will display the file extension as a 'badge' over the thumbnail.

UTI declarations within installed apps will override any declarations within Quick Look generators,
so if users have apps with conflicting information, the badge may not display.