Let Cinnamon fallback to Cinnamon 2D

Looks like no one ever changed the fallback of Cinnamon since it got forked from Gnome which means Cinnamon falls back to gnome-fallback if no 3D acceleration is available. At least I wasn't able to find any previous discussion or reasoning why this would be desired... ;-)

This pull request includes 1 commit that changes the fallback from gnome-fallback to cinnamon2d.