Modernise add-on for Vaadin 8

[maxschuster]

This add-ons API is quite different from Vaadin 8s API.

I would like to introduce event listeners, as I know them from the Framework:

Idle idle = Idle.track(ui, 5000);
idle.addUserActiveListener(e -> status.setValue("You are now active"));
idle.addUserInactiveListener(e -> status.setValue("You are now idle"));

Also the javascript part of this add-on has some polyfill code thats not required for officially Vaadin 8 supported browsers anymore. I would like to remove it.

As a training I have made a fork of this project where I made those changes and also created some tests for it. 😄

[maxschuster]

Any chance that this or #5 will get merged?

[maxschuster]

Thanks for merging the PR 👍