Better user interface for composite signals

[bindreams]

The composite signal creator needs to be improved. Things that still need to be added:

• [ ] A visual interface for building expressions, similar in spirit to https://www.desmos.com/scientific,
• [ ] A warning when a connected signal is not used in the expression,
• [ ] An error when a non-connected signal is used,
• [ ] An option to connect a relevant signal when its name is used in the expression,
• [ ] An error when the expression is invalid or unknown variables are used.