bookmarks: introduce a bmstore to manage bookmark persistence Bookmarks persistence still showed a fair amount of its legacy as a monkeypatching extension. This encapsulates all bookmarks serialization and parsing in a single class, and offers a single location where other bookmarks storage engines can be substituted in. As a result, many files no longer import the bookmarks module, which strikes me as an encapsulation win. This doesn't do anything to the current bookmark state yet, but I'm hoping put that in the bmstore class as well.