Support legacy framesets when applying templates.
The <frameset> and <frame> elements are obsolete and non-conforming in HTML 5, but some really old content still uses them. Because <frameset> is used instead of <body>, the current Guise Mummy implementation won't pick it up to be used as the content element, resulting in a warning like this:
Improve content element discovery during template application to find framesets and place them in the template. Recognition of frame links via the src attribute needs to be added as well, so that the files can be relocated for template application and for general renaming.
Note that the tricky part will be in switching the element type (but keeping the template attributes) rather than just inserting the content, as the template will be using <body> and the current logic simply inserts the content children.