Clean up styles 2nd half

As I mentioned in #869 I split up my style cleaning because it felt just a little too big for one PR. This branch was created from my clean_up_styles branch, so I'd like to merge them together more or less. Please let me know if that's the wrong approach :)

The besides the normal changes from the last PR, I changed a little bit about check boxes work. I realized that  radio was overwriting its styles and fixed that. Due to these changes I'd be interested to discuss the possibility of now splitting out check into its own file since it's not a simple call to its super class any more.

**UPDATE** 
Now that I looked at the inline comments, I see better the implications of branching from a branch. Please advise :grin: 
I don't get your argument here. `style_with` is a class method, hence it is executed at the time the file is loaded. Therefore it is always called before initialize. E.g. one time when we first load the check file.

Calling `style_init` after the `@app` creation in the superclass (`CheckButton`) should also be no problem. Therefore I'd rather like it if as much as possible happens in the super class - so we don't have any duplication.

Maybe I'm just missing the problem, though.