New popup widget

This will make the popup with callout functionality easy to reuse elsewhere - in the first case most likely the popup menus for the category widget.

*.php
* Added links to the new widget

ve.ui.Context.css, ve.ui.Widget.css
* Moved styles to the widget stylesheet

ve.ui.Context.js, ve.ui.PopupWidget
* Moved "popup" specific stuff to the new popup widget

Change-Id: I823c6e2c5e1ec11088898e9621d93e983c3b76f3
Merge "New popup widget"