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
private String name;
	private IConfigurationElement configElement;
	private int priority = Priority.NORMAL_PRIORITY_VALUE;
	/**
	 * This is calculated based on the priority and appearsBeforeId when all of the descriptors
	 * are first loaded. This is what's used to sort on after that.
	 */
	private int sequenceNumber;
	
	private String appearsBeforeId;
	private Expression enablement;
	private Expression possibleChildren;
	private String icon;
	private boolean activeByDefault;
	private IPluginContribution contribution;
	private Set overridingExtensions;
	private List overridingExtensionsList; 
	private OverridePolicy overridePolicy;
	private String suppressedExtensionId;
	private INavigatorContentDescriptor overriddenDescriptor;
	private int hashCode = HASH_CODE_NOT_COMPUTED;
	private boolean providesSaveables;
	/**
	 * Creates a new content descriptor from a configuration element.
	 * 
	 * @param configElement
	 *            configuration element to create a descriptor from
	 * 
	 * @throws WorkbenchException
	 *             if the configuration element could not be parsed. Reasons