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.
reworked menu system

corrected variable import and restructed the SCSS
Added check to ensure that any specified height is overriden before calculating the heights.
Merge pull request #3675 from thomasgohard/fix-equalheight-override-static-height

[Equal heights] Override specified height before calculating heights
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.
Merge pull request #3668 from masterbee/menuadjustment

MENU: Reworked logic flow and optimized imports
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
Fixed the markup for the share widget's overlay.
Merge pull request #3686 from thomasgohard/fix-share-widget-overlay

Fixed the markup for the share widget's overlay
Fixed #3688 by only returning false for known controls
Tabs: Switched tabpanel section elements to duv elements to resolve validation and HTML5 outline issues
Merge pull request #3690 from pjackson28/tabsswitchsectionstodivs

Tabs: Switched tabpanel section elements to duv elements to resolve validation and HTML5 outline issues
Merge pull request #3689 from LaurentGoderre/fix-3688

Fixed #3688 by only returning false for known controls
Removed wb-inv from fourth column on working examples page