CologneBlue rewrite: output the sidebar items as <ul> lists

This fixes the second part of bug 41456.

We also wrap each portlet in a <div> with class=portlet and an id like in
other skins (p-<heading-name>).

These changes should make CologneBlue more compatible with gadgets
and user-scripts that expect standard sidebar structure to add new links
to it.

Change-Id: I51c022bf22b881d4bd33c8d0e36c3e3226ac8b8f