Convert CommonMethods to Common::Element

Addresses the actionable work from #758.

In addition to a more descriptive name, this also broke up the various methods in `CommonMethods` into more descriptive modules--`Common::Positioning`, `Common::Visibility` and `Common::Remove`.

FWIW, I also tried briefly to create a `Common::ArtElement` for those element types, which would have encapsulated `Common::Style` and `Common::Clickable` (which all the art stuff shares). It had a problem, though, since the meta-programmatic addition of `style_with` to the class the module's included in relies on the `Common::Style` being included directly into the class itself. We could either change the pattern somehow to make the `style_with` instance level and called within `initialize`, or just let that particular idea go for now... I'm actually inclined toward the later.