AbstractCachingWebPanelRenderer does not clean up correctly on plugin disabled

This mainly affects the build of ondemand as this bug causes memory leak during xml restore.

AbstractCachingWebPanelRenderer implements DisposableBean to unregister itself from EventManager during plugin shutdown. But since the web-panel-renderer is not managed by Spring, the destroy() method is never called.
it's managed by WebPanelRendererModuleDescriptor, which should overwrite the destroy() method in AbstractModuleDescriptor and ask the AbstractCachingWebPanelRenderer to unregister itself and also clear its cache.