Hide or disable "cancel" button on widget editor when opened on drop

When dropping a widget that holds required fields (like a layout or content view widget for instance), the widget editor is opened but it's still possible to hit the "cancel" button.

As there's no validation on required field currently, user will not be warned that parameter is required, and an error will be produced on Nuxeo side.

=> disable or hide the "cancel" button in that case only