Implemented DiagramEditor resource disposal/reinitialization based on Eclipse IPartLi...

Implemented DiagramEditor resource disposal/reinitialization based on Eclipse IPartListener2
partHidden/partVisible callbacks.

The important developer-visible changes are:
* Diagram editor  should now use the class org.simantics.modeling.ui.diagramEditor.DiagramEditor as the editor class.
* DiagramEditor extension class attributes require a *viewer=clazz* argument after the *org.simantics.modeling.ui.diagramEditor.DiagramEditor:* class string, where clazz is something inherited from *org.simantics.modeling.ui.diagramEditor.DiagramViewer.
* DiagramViewer.getInputValidator() is now public
* Eclipse command handlers or any other code comparing EditorPart instances to *org.simantics.modeling.ui.diagramEditor.DiagramViewer* must be changed to compare to *org.simantics.modeling.ui.diagramEditor.DiagramEditor* and get the viewer through DiagramEditor.getViewer().

fixes #3213

git-svn-id: https://www.simantics.org/svn/simantics/sysdyn/trunk@24212 ac1ea38d-2e2b-0410-8846-a27921b304fc