Improve DeferredClasspathContainerInitializerJob.belongsTo

- Add ClasspathContainerInitializer.class to the test for families in
belongsTo to allow a client to wait for classpath container
initialization without knowing or caring about PDE specifically.

Author: merks

ui/org.eclipse.pde.core/src/org/eclipse/pde/internal/core/RequiredPluginsInitializer.java

@@ -104,7 +104,7 @@ protected IStatus run(IProgressMonitor monitor) {
 
 		@Override
 		public boolean belongsTo(Object family) {
-			return family == PluginModelManager.class;
+			return family == PluginModelManager.class || family == ClasspathContainerInitializer.class;
 		}
 	}