New gallery processor should have its own plugin class

For CMS7-4348 a new gallery processor was created that saves width & height of images. This new processor should replace the old gallery processor in new installations of the CMS.

However, the new gallery processor is instantiated by the same plugin as the old gallery processor: the class org.hippoecm.frontend.plugins.gallery.GalleryProcessorPlugin. An old vanilla installation of the CMS that is upgraded to the next version will therefore automatically used the new gallery processor. This will break uploading of images, as the new gallery processor expects a new node type and additional configuration.

To fix this issue, the old plugin class should be restored, and a new plugin class should be added to instantiate the new gallery processor. The new default configuration can then use the new plugin class, while old configurations can keep using the old plugin class.