Image insertion

Objective:

* Allow inserting images from local wiki and commons

Changes:

ve.init.mw.ViewPageTarget.js
* Add media insert button to toolbar

ve.init.mw.Platform.js
* Add getMediaSources method - defaults to local wiki and commons

ve.ui.MWMediaInsertDialog.js
* New dialog for inserting media
* Uses a media select widget and inserts block images

ve.ui.Dialog.css
* Added styling for media select widget in media insert dialog

ve.ui.Widget.css
* Added styles for media select widget and media select item widget

ve.ui.MWMediaInsertButtonTool.js
* New tool for inserting media

ve.ui.MediaSelectItemWidget.js
* New item widget for media select widgets

ve.ui.MediaSelectWidget.js
* New widget for searching for and selecting media items

ve.ui.TextInputWidget.js
* Added isPending method

VisualEditor.i18n.php
* New messages for media insert dialog

VisualEditor.php
* Added links to new files and messages

PhantomJS--

Change-Id: Ia803ff3ef518782ce76802d2dab7559686a1bb0a