Thinking about any lingering API inconsistencies we should fix (other than flattening graphic features) before v1.

• ImageWidget.managed_graphics -> ImageWidget.graphics
• ImageWidget.set_data -> make data a settable property, it resets the dim indices to 0. If the user wants to get back to the same frame they were at, they can do that explicitly afterwards. Similarly for vmin, vmax it resets the (vmin, vmax), the user can set it explicitly afterwards if they want.