Add missing styles for the Plone Classic theme.

The Plone Classic theme doesn't have the grid-based styles present in
the Sunburst theme. The styles added in this commit allow the
grid-based styling used by the panels to work when the theme in use is
Plone Classic or one based on it.
Activate the grid styles for the panels.

Without this activated, the preferences of the control panel configlet
won't be respected, and if the Plone Classic theme is used (or one
based on it) the panels will display properly.
Upgrade step adding new grid stylesheets.

For this we need to increase the metada version of the package, and
create an specific upgrade step.
Merge remote-tracking branch 'origin/grid-styles-for-classic-theme-v2'
Updated changes.