Steps can only take px width values, not ems

The steps frontend can only take px values for the step width (only the number without the px denominator), so it is not currently possible to have em based repsonsive layout steps. Given that breakpoints and grids both support em based specification, only the frontend is the missing piece to support this:

responsive-admin.js:

    breakpointList.push({
      'label': Drupal.settings.responsiveLayout.defaultBreakpoints[breakpointIndex].admin_title,
      'machine_name': name,
      // @todo: make sure that em/px based breakpoints work alike.
      'breakpoint': parseInt(Drupal.settings.responsiveLayout.defaultBreakpoints[breakpointIndex].width),
      'grid': Drupal.settings.responsiveLayout.defaultBreakpoints[breakpointIndex].grid_name,
      'regions': overrideList,
    });