When reading field definitions check the plugin class not the field definition

When reading editor_templates nodes of a document type definition, now only nodes that have a property "field" defined are selected and shown. This is not entirely correct. Also the value of the plugin.class property must be taken into account. If that is either one of:

• org.hippoecm.frontend.editor.plugins.field.PropertyFieldPlugin
• org.hippoecm.frontend.editor.plugins.field.NodeFieldPlugin

then there must be a field property also, and together they define the supported field.

Now it is possible that a different plugin class is defined and also a field property, and those definitions don't work correctly. Such a definition must be ignored and reported as an unsupported field. The users must see a message saying that 'not all fields are supported'.