Allow open ui to set "You have unpublished changes" exclamation icon in Experience Manager

Use case is that an openui plugin triggers backend code to update preview nodes via jcr api. There are unpublished changes, but Experience Manager does not recognize them. One trick is to lock one of the containers within the page programmatically in the backend. Then the exclamation mark shows.

Note that without doing any locking tricks openui's 'ui.channel.refresh' or 'ui.channel.page.refresh' methods don't make the exclamation mark appear