Add a family to UpdateClasspathsJob

In some cases it is good to wait for a certain type of jobs to complete,
this now adds a family to UpdateClasspathsJob so it can be used to wait
for such kind of jobs to complete (e.g. used in Tycho
api-analysis-mojo).

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

@@ -89,6 +89,11 @@ public UpdateClasspathsJob() {
 			setRule(ResourcesPlugin.getWorkspace().getRoot());
 		}
 
+		@Override
+		public boolean belongsTo(Object family) {
+			return family == PluginModelManager.class;
+		}
+
 		@Override
 		protected IStatus run(IProgressMonitor monitor) {
 			try {