CVCenter: rather use \UI.asClass.notNil to check if UI.registerShutDown or ShutDown.add should be used

Signed-off-by: nuss <st9fan@gmail.com>
Add GUI-elements to the 'actions' tab in CVWidgetMSEditor.
Bugfix in CVWidgetMS: an editor should only be opened
once when clicking 'actions'

Signed-off-by: nuss <st9fan@gmail.com>