Added a mechanism to filter extension points as they are discovered.

I couldn't quite find one place to do this, so it's spread apart
somewhat.

core/src/main/java/hudson/ExtensionComponent.java

@@ -24,7 +24,9 @@
 
 package hudson;
 
+import hudson.model.Describable;
 import hudson.model.Descriptor;
+import jenkins.ExtensionFilter;
 
 /**
  * Discovered {@link Extension} object with a bit of metadata for Hudson.
@@ -32,6 +34,8 @@ import hudson.model.Descriptor;
  *
  * @author Kohsuke Kawaguchi
  * @since 1.356
+ * @see ExtensionFinder
+ * @see ExtensionFilter
  */
 public class ExtensionComponent<T> implements Comparable<ExtensionComponent<T>> {
     private final T instance;
@@ -66,6 +70,15 @@ public class ExtensionComponent<T> implements Comparable<ExtensionComponent<T>>
         return instance;
     }
 
+    /**
+     * Checks if this component is a {@link Descriptor} describing the given type
+     *
+     * For example, {@code component.isDescriptorOf(Builder.class)}
+     */
+    public boolean isDescriptorOf(Class<? extends Describable> c) {
+        return instance instanceof Descriptor && ((Descriptor)instance).isSubTypeOf(c);
+    }
+
     /**
      * Sort {@link ExtensionComponent}s in the descending order of {@link #ordinal()}.
      */