Allow only one instance of the Model Quality view part

* The model quality services registers an update listener with the
  view part's (static) controller. Therefore, only a single instance
  of the view part can be managed

Issue-Ref: 4310
Issue-Url: af3#4310



Signed-off-by: Simon Barner <barner@fortiss.org>

org.fortiss.tooling.ext.quality.ui/plugin.xml

@@ -8,14 +8,14 @@
             contributor="org.fortiss.tooling.ext.quality.ui.ModelQualityExtractionMenu">
       </contextMenuContribution>
    </extension>
-   <extension
-         point="org.eclipse.ui.views">
+   <extension point="org.eclipse.ui.views">
       <view
             class="org.fortiss.tooling.ext.quality.ui.view.fx.ModelQualityFXViewPart"
             icon="icons/metric.png"
             id="org.fortiss.tooling.ext.quality.ui.metricsView"
             name="Metrics"
-            restorable="true">
+            restorable="true"
+            allowMultiple="false">
       </view>
    </extension>
 </plugin>
\ No newline at end of file