Enhance the IContributionFactory to filter contributions

Currently there is no way for filtering a contribution to the model on a
global level.

This adds a new method to IContributionFactory#isEnabled that allows
extenders to mark a given contribution as being disabled.

See eclipse-platform/eclipse.platform.ui#2217

runtime/bundles/org.eclipse.e4.core.services/src/org/eclipse/e4/core/services/contributions/IContributionFactory.java

@@ -16,8 +16,10 @@
 import org.eclipse.e4.core.contexts.IEclipseContext;
 import org.osgi.framework.Bundle;
 
-// TBD this became an utility method to create object from a bundle.
-// Change it into an utility method somewhere.
+/**
+ * {@link IContributionFactory} is responsible to create contributions and check
+ * if a contribution is enabled
+ */
 public interface IContributionFactory {
 
 	Object create(String uriString, IEclipseContext context);
@@ -26,4 +28,8 @@ public interface IContributionFactory {
 
 	Bundle getBundle(String uriString);
 
+	default boolean isEnabled(String uriString) {
+		return uriString != null;
+	}
+
 }