added urldecode on file names to deal with special character, e.g., File:Kansallisarkisto-Susanna_Ånäs-Dan-nl.xml
added in preview function, ticket nr #145
re-factored __constructor placement to the top of the class
re-factored the back to form links
re-factored wfMessage()->plain() to wfMessage()->escaped()
re-factored code so that it uses ContentHandler
  * metadata is now stored as content in namespace GWToolset: instead of in the namespace File:
  * mappings are now stored as content in namespace GWToolset: instead of the main namespace

Change-Id: I209244c9bf5fe9f2f1eb89eea0cc1d91d3fd815f