Fixed issue #21: "Placement Exceeds Your Reach" message displays when a block is placed in survival with 0 range upgrades.
Fixed issue #22: Sometimes two blocks are placed at once when in regular build mode.
Fixed mouse clicks in Modifier Settings sometimes being in the wrong place.
Fixed typing in multiple text fields at once.
Added diagonal line, wall, slope floor and cube icons.
Added undo, redo, replace and 'open modifier settings' buttons. (Only last 2 work for now).