Add option to the FieldGroup annotation for the state of the group

The component editor in the side panel shows the properties in collapsible groups. By default their initial state is to be expanded. We can add an option for developers to configure this state. They can then decide if a group should be shown collapsed or expanded when the editor is opened the first time.

For example the main properties can be in the first group which is shown expanded, and more advanced and less used properties can be put in a different group which is collapsed by default. This can help giving the users focus on the important or most used properties.

For this the org.hippoecm.hst.core.parameters.FieldGroup should get a new boolean property "expanded" which by default is true.