Add a way to modify SimVar parameters

[MikeRomaa]

Summary

We need a way to modify the parameters for a SimVar slider. This includes:

• switching between string, boolean, and numeric values
• modifying the slider range for numeric values
• modifying the variable's unit field

References

ACE v2 had a system for doing this through another popout menu. I think this could be cleaner, but its a decent start.