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 keyboard-driven workspace shifting up and down to work, you need to apply linuxmint/Muffin#69, or you will be stuck on the current row forever.**

This patch includes my Expo-workspace-thumbnail-zoom patch ("expo-sure").