abstracts styles that recur in different objects

There are many cases in Field Day of multiple instances of an object having the same style (e.g. interval buttons, or buttons that are identical across portrait and landscape modes). In the past we've implemented styling on a per-instance basis. This abstraction will make the app easier to build, maintain, and design.

I'm immediately merging because all changes in this merge request affect only XML files describing what objects look like, and this is easily verified. No functionality is changed.