Issue #130: Remove the global theme settings page

Additional PR for https://github.com/backdrop/backdrop-issues/issues/130.

This PR removes the remaining parts of the global theme settings page and removes it entirely. Themes that do not provide any settings do not get a settings page at all.

Because this dealt heavily with saving theme settings into configuration (instead of variables), this PR also converts Color module to read and write from the new config files instead of using the old variables. Upgrade path is provide to convert color settings into the theme config.
This change should be squashed into a previous commit, or removed.