Review dialog classes

We should add an interface for dialog classes that are passed to simplePlugin::openDialog, and document this and associated classes.

We should split execute and save_object as is done for other FusionDirectory classes.

Related to this is the problem of submitting form with enter: since 54220789 it submits a special hidden input called default_submit_hidden_button, but this is not correctly supported by dialogs which expects either add or cancel button to be pressed. We could map default_submit_hidden_button to add, but I’m afraid in some cases it may be active at the first dialog load? There is a PID system for management classes but only for them, maybe it would be useful to have one for other classes but I’m not convinced.