Expose the breakpoint variables in the wizard

[liv]

This is an interim post, cos I've found in the past that not documenting bugs until they're actually finished is not as helpful or organized as I expected it might be.

I'm working on Expose the breakpoint variables in the wizard. Except that the discussion of what I actually need to achieve this is scattered over several different Github issues and some IRC chat, so if I can put everything together in one place I am more likely to know what I'm doing.

This is the discussion I'm primarily working from:
Investigate breakpoint of layouts. However, see chat with fu (pasted in as a comment) for further detail on this.

Apart from: Add two sets of breakpoint variables

Need to build my patch on: use generate_medium_media_query()

More discussion in dw_styles: Mobile styles redux and in logs from the recent dev chat