Bootstrap3 : IconType refactor

Here's the PR for #274.

I've not drop the "implements Invertible<Icon>" yet because it would change many classes (BootstrapLink, BootstrapButton, ...).
Will do it later.

Also some class where using as dummy IconType call IconType.NULL. I've replaced it with a "null" value.
I'm using it in my main project. Everything seems to be fine!