Make use of the list of Atoms in the class attribute in selector matchin...

...g.

Fixes #3111
When I added the has_class function in the TElement trait (so it would be available to be implemented in impl<'le> TElement for LayoutElement<'le>, it meant I also add to implement it here. Ideally I would not implement this here but I'm not sure how to achieve that. 

Since I had to implement I thought of using a simple pass through to has_class but it fails since there is 2 implementations of has_class in this file. To make the code compile code compile I have written this dummy function.

Hopefully someone can suggests something for this in the PR and I can update the PR.