More options for profile curve definition

[stfnp]

Add more options for defining the line, arc and spiral segments, especially in terms of cartesian coordinates and angles.
Something like this, depending on segment type:

• X Position [Relative/Absolute]
• Y Position [Relative/Absolute]
• Angle [Relative/Absolute]
• Length [Relative/Absolute]
• Curvature [Relative/Absolute]

Determining the segment from these constraints requires (numerically) solving systems of eqations and might not always lead to a valid solution (for example, it's not possible to define an arbitrary x coordinate for the endpoint for a vertical line). So errors have to be dealt with in a way that is transparent to users. (Idea: Show error message as red text at the top or bottom of the plot)