Tether beta-0.12

• Fixed crash when attempting to resimulate a cable that has too few spline points, now resimulating requires at least 2 points
• Fixed tangents on adjacent points being lost when adding a point with the Unreal spline tools
• Can now set normal-aligned tangents by holding alt when placing a point, even when left mouse button is not down
• Removed the ability to realtime simulation preview on multiple selected cables at the same time, as it's not useful and can lead to inconsistent results