Added defense level slider to Sandbox Mode. A higher defense level equals more durable buildings and more capable obstacles
Added saving to Sandbox Mode. Currently allows saving one custom configuration - terrain, buildings, obstacles, and defense level - which is then loaded automatically on the next visit to Sandbox Mode
Right-click to cancel stops any in-progress aiming but no longer deselects the current disaster (so it works the same as dropping an unwanted disaster in the deselect tray)