klighd.piccolo: enabled installation of customized tooltip renderer i…

…n sub classes of 'PiccoloViewer', as demanded in #73

plugins/de.cau.cs.kieler.klighd.piccolo/src/de/cau/cs/kieler/klighd/piccolo/viewer/PiccoloViewer.java

@@ -144,7 +144,7 @@ public PiccoloViewer(final ContextViewer theParentViewer, final Composite parent
         this.magnificationLensVisibleSupplier = installEventHanders(camera);
 
         // add a tooltip element
-        new PiccoloTooltip(canvas);
+        this.installToolTip(canvas);
 
         // A timer being in charge of buffering and thus aggregating a bunch of single
         // view transform changes occurring closely after each other to a single view
@@ -223,6 +223,15 @@ protected Supplier<Boolean> installEventHanders(KlighdMainCamera camera) {
         return () -> magnificationLensHandler.isLensVisible();
     }
 
+    /**
+     * Installs the tooltip support on this diagram viewer, clients may override.
+     * 
+     * @param canvas the {@link KlighdCanvas} to install the tooltip support on
+     */
+    protected void installToolTip(KlighdCanvas canvas) {
+        new PiccoloTooltip(canvas);
+    }
+
     /**
      * Provides the visibility state of the magnifying lens.<br>
      * This is evaluated by particular Klighd...EventHandlers that are inactive in case the