Scroll into view support

Objectives:

* Scroll when needed to show highlighted (with keyboard) or selected (by
  any means) options in select widgets
* Allow clipping and automatic scrolling for certain elements when they
  are otherwise going to be rendered partially out of view

Changes:

*.php
* Add links to new file

ve.ui.Widget.css, ve.ui.Dialog.css
* Removed unneeded x-axis overflow rules

ve.ui.ClippableElement.js, ve.ui.Element.css
* New mixin, adds visible area clipping support to an element

ve.ui.PopupToolGroup.js, ve.ui.MenuWidget.js
* Mixin clippable element

ve.ui.OptionWidget.js, ve.ui.OutlineItemWidget.js
* Add scroll-into-view configuration for option widgets

ve.ui.SearchWidget.js
* Scroll items into view when highlighting with keyboard

ve.Element.js
* Add getBorders, getDimensions, getClosestScrollableContainer and
  scrollIntoView static methods
* Add getClosestScrollableElementContainer and scrollElementIntoView
  methods

Bug: 53610
Change-Id: Ie21faa973a68f517c7cfce8bd879b5317f536365
Merge "Scroll into view support"