Create an undo level for 3d mode editing

When exiting 3d mode, all edits made while in 3d mode are rolled into a single '3D Mode Editing' undo level
Add context menu to undo history list

Can undo/redo to the selected item on the list. Also reversed the direction of the list to go top->bottom
Merge branch 'master' of https://github.com/sirjuddington/SLADE.git