Save as keeps the old model reference in aird file

If a model is saved using "save-as", the old model remains in the .aird file. This implies that the old model is loaded as well when the new model gets loaded. It also implies that the old diagrams appear on the welcome page. If the user does not pay attention, he is looking at an old diagram. Worse, the diagrams in the old files are rewritten during a save of the new model if they are opened (likely related to the issue that they are dirty due to an initial Sirius action, see issue #49). As a workaround, the .aird file can be deleted if only GMF files are used. Otherwise, additional "semanticResources" can be removed manually from the file.

Example aird content after saving a model file using "SaveAs" with the added "2" postfix

  <semanticResources>TestSaveAs.uml</semanticResources>
  <semanticResources>pathmap://UML_LIBRARIES/UMLPrimitiveTypes.library.uml</semanticResources>
  ...
  <semanticResources>TestSaveAs2.uml</semanticResources>