Hook up responsive prototype viewing to variable modes

Since variables with support for modes was introduced, we followed one of the examples to create a single instance for multiple breakpoints. Using modes, we can switch any frame to the desired breakpoint automagically; awesome!

Now that responsive viewing is added to prototyping, it would be really helpful to be able to link our breakpoint modes to this viewing experience so that the prototype starts on the correct breakpoint depending on the viewers screensize.