Add a Spinner control for desktop and web projects

Add a built-in Spinner control to desktop and web projects for showing that an operation is still running when its completion percentage is unknown. It should be a compact circular animation, available in both visual designers, with simple activation and colour properties. Developers should not need to supply an animated GIF or implement their own drawing and timer code.

An indeterminate ProgressBar remains useful for larger layouts; a Spinner fits beside a button, label, or other compact content.