added an auto-discovery mechanism for extension points (and most notably Descriptors.)

- extensions are now stored in ExtensionList.
- discovery is extensible, but by default we use Sezpoz for annotation-based injection. This requires JDK6 for build.
- a care has been taken to preserve backward compatibility. DescriptorList and manually adding to it continues to work.
- AdministrativeMonitor is converted to use the new approach entirely, as a guinea pig of this feature.
- Descriptors for View is partially switched to this new approach, where some classes register automatically while others do manually. This tests the compatibility aspect of this feature.

A pluggable discovery is an attempt to embrace Plexus integration work, although for that to really work we need more coding.

Next stop, more tests.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@15601 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/ExtensionPoint.java

@@ -29,10 +29,16 @@ package hudson;
  *
  * <p>
  * See respective interfaces/classes for more about how to register custom
- * implementations to Hudson.
+ * implementations to Hudson. See {@link Extension} for how to have
+ * Hudson auto-discover your implementations.
+ *
+ * <p>
+ * This interface is used for auto-generating
+ * documentation.
  *
  * @author Kohsuke Kawaguchi
  * @see Plugin
+ * @see Extension
  */
 public interface ExtensionPoint {
 }