Fix null reference exception on disposing EDM View.

When an exception is thrown when loading a .edmx file
the EDMDesignerViewContent's Dispose method was not
handling the EDMView being null and causing the
underlying exception to be hidden.
Reformat code.
Fix null reference when creating new EF data model file.

Do not add an EntityTypeMapping element if it already exists.
Fix intermittent error were the EF data model view content
does not have a workbench window when the file is renamed
on save.