Add block docking functionality to the Classic theme

The block docking functionality available in the Clean theme, or an alternative that fulfills similar requirements, needs to be implemented for the Classic theme.

Use the previous Clean block docking functionality, along with user feedback, to determine requirements. Then look at the old Clean (YUI) code for some guidance on what needs to be considered (eg handling of block height), keeping in mind some of it will be outdated and no longer required (eg unsupported browser handling).

Edit: It is important to understand what problems are solved by the feature, before determining whether the entire functionality as it existed in Clean is required, some derivative thereof, or some new feature, to ensure the greatest user benefit.