Stringer Mass is not saved in craft

[uriyuzhui]

For tanks and even decouplers, stringer mass is always set to current/best value,
There is a slider for it, but it does not seem to actually set any value.

Steps to reproduce:

1. Make a tank
2. Use the slider to set a higher-than-lowest stringer mass value
3. Save the craft and reload it
4. The stringer mass is reset to best value