Added ability to update diagram from last used view

As an extension to issue hallvard#93, this commit introduces
changes to allow updating the diagram based on the
view that PlantUML last used (i.e., the currentPart)
instead of the view that is currently in focus. To make
this work, PlantUML now ignores its own view (i.e., does
not set the view in which it renders the diagram as the
current part) and has a new method
updateDiagramTextFromCurrentPart() which updates the
diagram based on the currently set currentPart.

bundles/net.sourceforge.plantuml.eclipse/src/net/sourceforge/plantuml/eclipse/views/AbstractDiagramSourceView.java

@@ -347,6 +347,9 @@ public void run() {
 	private IWorkbenchPart currentPart;
 
 	private void handlePartChange(final IWorkbenchPart part) {
+		if (part.equals(this)) {
+			return;
+		}
 		if (currentPart != null) {
 			currentPart.removePropertyListener(diagramTextChangedListener);
 		}
@@ -390,9 +393,17 @@ public void run() {
 			}
 		};
 	}
+
+	/**
+	 * Updates the diagram text using the part that was last used by PlantUML.
+	 * That makes it possible to programmatically update the current diagram.
+	 */
+	public void updateDiagramTextFromCurrentPart() {
+		updateDiagramText(true, currentPart, null);
+	}
 
 	/**
-	 * Updates the diagram text from the current part (if linked to it) and the current selection.
+	 * Updates the diagram text from the part that is currently active in the IDE (if linked to it) and the current selection.
 	 */
 	public void updateDiagramText() {
 		updateDiagramText(true, null, null);