Insert Reference

Objective:

* Allow opening reference dialog with arbitrary selection
* Auto-insert reference when selection is not a reference node

Changes:

ve.init.mw.ViewPageTarget.js
* Added reference button to toolbar

ve.init.Target.js
* Add getToolbarSubset so we can exclude the reference button from
  the toolbar in the reference dialog (nested references are not
  allowed).

ve.ui.MWReferenceDialog.js
* Stop storing referenceNode (not needed)
* Only store internalItem on open if there's a focused node that's a
  reference
* Use wrapper paragraph when creating a new reference
* Create new reference on dialog close if required

ve.dm.InternalList.js
* Major rewrite to support key less references.
* Add new method for creating a transaction to insert a new iternal
  item. Also returns the index of the new item to be passed to the
  reference node.

Change-Id: I839ae165c299248484ce93d4ab087318a95fbb94