Taranta has max size of dashboard that can be set by designer

Because the time and the machine on which a dashboard is developed and is run might differ, also the screen size might differ. But currently Taranta editor does not allow the user to set max width and max height of the running dashboard. The consequence is that the developer can only guess if the placement of a certain widget will be displayed in the target screen without having to scroll the window.

So make it possible in the editor to specify a property of the currently edited dashboard that is "max-size" (or max-width, max-height). Once that is done, then the canvas background should make it clear to the user where the 'usable' area of the screen is. 

In the editor, widgets can still be placed outside the selected area; when running the dashboard on a window that has such size, those widgets will require horizontal or vertical scrolling. So no dynamic resizing or layout responsiveness is needed. Essentially, this feature does not affect how dashboards are run - just like now.

The user might be to choose the max size from a menu of typical choices and also be able to specify a custom size (down to a reasonable minimum).

(This is also requested by Perentie: https://docs.google.com/document/d/18fgB3CU0OwqcyW9M6PETQT9MR6pTUwA52BQ_pbmV3q8/edit#heading=h.5iq48cqxiup7)