Reference dialog commingling

Objectives:

* Merge reference insert and edit dialogs
* Change workflow to put editing/creating a new reference first
* Add secondary page in dialog for selecting an existing reference

Changes:

*.php
* Cleanup unused files/messages

ve.ui.Dialog.css
* In the footer; make primary, constructive and destructive buttons
  appear on the right; all others on the left

ve.ui.MWReferenceSearchWidget.js
* Fix documentation
* Remove create option and reuse section header items

ve.ui.MWReferenceInsertButtonTool.js,
ve.ui.MWReferenceEditButtonTool.js,
ve.ui.MWReferenceButtonTool.js
* Merge reference button tools

ve.ui.MWDialog.css
* Remove body styles, use padded option of layout instead
* Update selectors as per merging of dialogs

ve.ui.MWReferenceInsertDialog.js
ve.ui.MWReferenceEditDialog.js
ve.ui.MWReferenceDialog.js
* Merge reference dialogs
* Add buttons to switch between edit and select mode

ve.init.mw.ViewPageTarget.js
* Update reference button name as per merging of tools

ve.ui.SurfaceWidget.js
* New widget!
* Encapsulates a "sub-surface"

Bug: 51152
Bug: 50458
Change-Id: I8265febf4fd8f64d2ac40470ff033bac68b24d99
Merge "Reference dialog commingling"