Follow-up for #5471
This makes the breakpoints option and `breakpoints()` method clearer and introduces the responsive option and `responsive()` method, which will turn on the breakpoints.
The return value of `currentBreakpoint()` was simplified to only ever return a string (empty if none).
Also, added convenience methods: `responsive()`, `getBreakpointClass()`, and `currentBreakpointClass()`.
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