Implement 'change' events in templates models

This will make generating live previews possible.

Changes:
* Add change events to template model.
* Set up connect/disconnect.

Also:
* Add missing fallback for getParameterLabel (Param#label is
  optional per the TemplateData spec).
* Implement getWikitext, to be used by the UI dialog later
  to create a preview from the wikitext.
* Correctly mark ve.dm.MWTransclusionNode#escapeParameter as
  being a static method.

Change-Id: Ie306ed03babf11568e954b1813ce5324f57d7f0e