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()`.
This adds a breakpoints option. By default, this option is false meaning this is an opt-in feature.
When passing true, it will use a default set of breakpoints. Or custom breakpoints can be passed if users do not like our breakpoints (or previously-existing style decisions).
- Add breakpoints option.
- Adds some new (currently unused) classes: vjs-layout-medium, vjs-layout-large, vjs-layout-x-large, and vjs-layout-huge.
- Add updateCurrentBreakpoint and currentBreakpoint methods to the player.
- Update css/components/_adaptive.scss
- Add sandbox/responsive.html.example
Closesvideojs/video.js#4371
