VisibilityCSSAdapter should implement the "Serializable" interface

[yurili8]

Firstly , I have to say it is really a great addon. But I do encounter one issue recently, which is a showstopper.

some classes, like "VisibilityCSSAdapter", are not implement the "Serializable" interface. it will fail in the mod cluster environment when switch the node to another. because the session could not be replicated.

Any answer for this will be appreciated.

[JarekToro]

I appreciate the comment. Right now Responsive-Layout is going to be in a PR mode only. I no longer have the time to fix these issues. But will try to accept PRs and upload to Vaadin Directory.