Add classes to the webform analysis table to allow it to look better in certain themes (like Seven)

This issue is a continuation of #1009208: Improve the understandability of the results -> analysis tab, but with a different approach.

The webform analysis table looks somewhat bad in the Seven theme. One of the main reasons is that Seven doesn't put any borders on its table rows, and given the way the table is constructed this leads the different sections to bleed into each other.

By adding a couple CSS classes, we can identify which rows in the webform analysis table are the beginning of a new question/response and therefore allow themes to highlight those however they want.

A patch for that is attached, as well as a before/after screenshot of what Seven would look like (well, in modern browsers at least) after applying the patch and adding this CSS:

.webform-analysis-report {
  border: 2px solid #BEBFB9;
}
.webform-analysis-report .webform-question {
  border-top: 2px solid #BEBFB9;
}

Not sure if it makes sense to add some CSS along those lines to this patch also, e.g. in a webform.seven.css or some such thing. (I don't know if Webform wants to get involved in providing theme-specific CSS, although it's something other modules do occasionally for popular themes.)