Use a trait to reduce code duplication in formatters

I was able to further reduce duplicate code by moving some parts of the viewElements() method into new trait methods. The slight differences between SvgImageFormatter and SvgResponsiveImageFormatter we noticed in 📌 Formatters shouldn't repeat core code Needs review today were respected, especially the cache tags handling 😉

Definitely a nice additional code factorization, thanks @mrshowerman!

Made a few tests, everything seems to work fine.

@grevil do you want to have a look at the MR before I merge it?

@mably go for it!

Merged!

Automatically closed - issue fixed for 2 weeks with no activity.