Connector/backend API polish: addable-object superclass, teach-panel promotion, getframe() #676

Small cluster of backend-API cleanup ideas, migrated from the wiki's Backends page TODO section.

• Objects addable to a backend (robots, shapes, etc.) don't share a common superclass with the abstract methods they need — tidy this up
• teach is still effectively a private/underscore-prefixed panel feature — promote it to a proper public API
• Add a getframe() method to Connector to return the current frame as a PIL image, for saving/embedding — needs a design that works across backends (PyPlot, Swift, ...) since none currently implement it