[Design] Enable multiple workspace rows (Real 2D workspace grid), add workspace-thumbnail zoom, and more

This patch enables multiple workspace rows, i.e., a real 2-dimensional workspace grid, fully supported by Expo and Scale. To navigate up and down using the keyboard, keep Ctrl+Alt+Up or Down pressed about 250 milliseconds; a shorter key-press will bring up Expo or Scale as usual.

To enable multiple workspace rows, open Cinnamon Settings, Workspaces.

**For optimum experience, you need a version of Muffin that includes linuxmint/Muffin#76.**

This patch includes my Expo-workspace-thumbnail-zoom patch, #1700.

Update 2013-11-09: This pull request has become quite old and it's getting increasingly difficult to keep synchronized with upstream development, so I will eventually be forced to either let it bit-rot or pull it.