Use a page action rather than a browser action

[stuartpb]

Looking at how things like Bookmarking are page actions (even though they apply to most pages), I'm thinking this clipper might make more sense as a page action (maybe even optionally including the ability to override Bookmarks as an option). It seems like Browser Actions (contrary to what it says in the docs) are more for extensions that don't entirely interact with the page's content.