Consider responsive breakpoint values being a theme setting

[TSP]

@Rikki: could you consider having the breakpoints for responsive be definable with a theme setting in default skins?

Otherwise I either have to copy all the CSS-rules within the core_responsive.css etc. into my own responsive CSS (and not include the default responsive) or change all the values within all the places where the responsive breakpoints are used. 

So wouldn't it be better if we could set these breakpoints from a theme setting? One for tablet and one for mobile. 

[Rikki]

Yeah, that's a good idea I'll definitely add that to a future release.