Stack panel, element mixins, cleanup

*.php
* Updated links to files

ve.ui.MetaDialog
* Added stack panel that now contains category and language editor panels
* Attached outline widget to stack panel

ve.ui.FlaggableWidget -> ve.ui.FlaggableElement
* Moved to elements

ve.ui.GroupWidget -> ve.ui.GroupElement
* Moved to elements
* Removed invalid event documentation

ve.ui.LabledWidget -> ve.ui.LabledElement
* Moved to elements

ve.ui.StackPanelLayout.js
* New class, mutually exclusive panel container

ve.ui.TitledPanelLayout
* Remvoed, using labeled element instead

ve.ui.Element.css
* Added for elements
* Moved label style here, from widget styles

*.css, ve.ui.ButtonWidget.js, ve.ui.InputLabelWidget, ve.ui.OptionWidget, ve.ui.SelectWidget
* Adjusted class names to reflect widget -> element migration

Change-Id: I32f504c844dba7aae1b286eef06ca046627bdc8d
Merge "Stack panel, element mixins, cleanup"