Bug 287139 [CommonNavigator] Add core expression to determine initial activation for NCE
Build ID: 20090619-0625 Steps To Reproduce: Currently, the definition of the Navigator Content Extension via plugin.xml allows to specify the boolean attribute allowByDefault which controls if the contributed content is active by default. It makes sence to change the type of the attribute from boolean to core expression, which will allow to evaluate the default context and show content depending on it. This will ease the control of content and enhance the declarative configuration of CNF and NCEs. More information:
bundles/org.eclipse.ui.navigator/src/org/eclipse/ui/internal/navigator/extensions/NavigatorContentDescriptor.java
* Determine if this content extension would be able to provide children for
	 * the given element.
	 * 
	 * @param anElement
	 *            The element that should be used for the evaluation.
	 * @return True if and only if the extension is enabled for the element.
	 */
	public boolean isTriggerPoint(Object anElement) {
		if (enablement == null || anElement == null) {
			return false;
		}
		try {
			IEvaluationContext context = NavigatorPlugin.getEvalContext(anElement);
			return (enablement.evaluate(context) == EvaluationResult.TRUE);
		} catch (CoreException e) {
			NavigatorPlugin.logError(0, e.getMessage(), e);
		}
		return false;
	}
	/**
	 * Determine if this content extension could provide the given element as a
	 * child.
	 * 
	 * <p>
	 * This method is used to determine what the parent of an element could be
	 * for Link with Editor support.
	 * </p>
	 * 
	 * @param anElement
	 *            The element that should be used for the evaluation.