Category UI improvements

Objectives:
* Ensure items don't get moved to the end when their sort-key is edited
* Add placeholder text and pending styling to input
* Auto-expand input to the end of the line
* Make the minimum input width smaller

Changes:

ve.ui.MWMetaDialog.js
* Added calls to fitInput on initialize
* Fixed sort key update and insert handlers to maintain item position when updating

ve.ui.GroupElement.js
* Added index argument to addItems, allowing items to be inserted at a specific location

ve.ui.PagePanelLayout.js
* Fixed CSS class name

ve.ui.StackPanelLayout.js, ve.ui.MenuWidget.js, ve.ui.SelectWidget.js
* Passed index argument through to group element

ve.ui.PanelLayout.js
* Fixed overflow direction for scrolling option

ve.ui.Inspector.css
* Moved border-box properties to text input widget class
* Set input widget within inspectors to be 100% by default

ve.ui.Layout.css
* Updated CSS class name
* Whitespace fixes

ve.ui.Widget.css
* Made text input widgets's wrapper default to 20em wide and the input inside it be 100%, using border-box to ensure proper sizing
* Adjusted category list item and input styles to make input appear more like a category item
* Whitespace fixes

ve.ui.MWCategoryInputWidget.js
* Made category input widget inherit text input widget, rather than just input widget

ve.ui.MWCategoryWidget.js
* Replaced group functionality by mixing in group element
* Added fitInput, which automatically make the input fill the rest of the line or take up the entire next line depending on how much space is left

VisualEditor.i18n.php
* Adjusted placeholder text for category input

Change-Id: I79a18a7b849804027473084a42c36133fdacad57
Merge "Category UI improvements"