Template and content adding

Objectives:

* Allow adding content or templates within a transclusion
* Add template placeholder to model and view which resolves to nothing
  when saving (thus disappearing if not resolved)

Changes:

*.php
* Add links to new messages and files

ve.ui.OutlineControlsWidget.js
* Organize controls into "adders" and "movers"

ve.ui.Widget.css
* Add styles for adders/movers sections of outline controls
* Make adders appear on hover to reduce clutter

ve.ui.Icons-*.css
* Add icon for "add item"

ve.ui.Dialog.css
* Add styles for add template fieldsets
* Make placeholder items in the outline italic

ve.ui.PagedDialog.js
* Pass adders config to outline controls

ve.ui.MWTransclusionDialog.js
* Add support for adding content and templates

ve.dm.MWTransclusionModel.js
* Add addPlaceholder method

ve.dm.MWTemplatePlaceholderModel.js
* New class, pretty much an empty part
* Using this makes the UI much easier to work with - no need to special
  case the outline control for new items
* Because it's not supported specifically in
  ve.dm.MWTransclusionModel.getPlainObject, it produces nothing and goes
  away naturally on apply

Change-Id: I3478560fb53ba2ccd3fb26bafb6a61e6415565eb