Optimised the overlays CSS.
Added overlay demo page.
Removed overlay-specific CSS from Share widget now that it's been included in the overlay CSS.
Toggle: refactored group toggle and fixed details click

* Group toggle code  now sets on/off CSS classes and data states
  on the grouped elements.
* event.target for details element clicks was referencing the nested
  summary element.  Fixed by passing reference to clicked details
  element.
Toggle: added Accordion example using group toggle
Details: added toggle.wb-details event

* New event allows toggle plugin to open/close polyfilled details
  elements without triggering a summary click.
* Events that open/close details elements are now allowed
  to propagate.
Added header to overlays.
Merge pull request #3681 from patheard/toggle-group

Toggle: group toggle updates and accordion example
Merge pull request #3683 from thomasgohard/plugin-overlay

Improvements to overlay plugin