Dashboard screenshot action

[fietser28]

I would like to have a way to have my dashboard generate screenshots (from a selected page or container of the dashboard?) so they can be saved to file or history (see #344).

I think a action to take the screenshot and output the image as a blob is the way to go. Inside the widget the user can select from a list of pages or containers the scope of the screenshot. I think containers are more flexible for this.