Media dialog support

*/index.php
* Added links to new files

VisualEditor.php
* Added links to new files
* Removed keys of non-existent messages

ve.ui.ContentDialog.js, ve.ui.MetaDialog.js
* Removed redundant comments

ve.ui.MediaDialog.js
* New dialog, just for media

icons.ai, picture.png, picture.svg, ve.ui.Icons-*.css
* Added picture icon

ve.ui.MediaButtonTool.js
* New button, just for media (shows up in the context toolbar)

ve.ui.DialogButtonTool.js
* New base class for dialog buttons

ve.ui.Context.js
* Added basic support for showing dialog buttons, in addition to
  annotation buttons, in the context toolbar - to test, select only an
  image node

ve.ui.Dialog.js
* Prevent clicks on the click-block from changing focus
* Moved initialize to below the event handlers and updated its
  documentation

ve.ui.DialogFactory.js
* Added a way to get the names of dialogs that can be used to edit a
  node

ve.ui.Inspector.js
* Removed close handler which set focus, this is done already in window

ve.ui.InspectorFactory.js
* Fixed comment so it's not telling lies anymore

ve.ui.Window.js
* Removed auto-focus on frame, it's changing the focus in the parent
  document which blows-away the focus in CE, and it really isn't needed
  as it turns out

VisualEditor.18n.php
* Added media dialog title message
* Added media tool tooltip message

Bug: 37870
Change-Id: I9150c46b3e292910fed899fa60d6da433049ca45