Harmonized Overlays, Lightbox, Share, Session Timeout and Modal

Harmonized the markup, CSS and JS for the 5 plugins to keep the overlays from each as consistent as possible..

TODO Further harmonize Session Timeout/Modal (or replace Modal) with the new modal approach included in Lightbox (and used by Overlays). @patheard Could you please look into this? The Lightbox plugin does this with the modal = true setting, buttons with class="popup-modal-dismiss" and a handler that listens for clicks on those buttons.

@masterbee This may impact your recent CSS overrides for the share widget.