Grid styles for classic theme v2

This is mostly for requesting review of the code, as I have based the work in the 1.5 tag that haven't been merged in master but it is the one I am using in production and triggered researching this issue.

These two commits fix the styling of the panels with the Plone Classic theme (or one based on it) and makes the style respect the options set in the control panel configlet in the Sunburst theme.

The current implementation includes the styles-classic-theme-grid.css in the CSS registry unconditionally, but that file is only available with the plonetheme.classic.browser.interfaces.IThemeSpecific browser layer, returning a 404 if not present. Maybe a non-issue, but couldn't think of a way to do it without this happening.

The difference with the previous version of these patches is that the grid style fixes for the classic theme are only applied to the panels, as there are documents using the same `row` and `cell` classes but depend on the styling done by the plain classic theme.