Line Lenght Constraint and Point to Point Constraint

Until now we had several ways of defining and displaying the distance between two points, with or without a constraint. Today I started changing the code so that we only use two elements for these:
- distance between two points constraint, which is effectively the current line length constraint
- the dimension function which displays the distance.

This way we'll be able to use the new constraint format and call the solver on all changes and display the length independently of the old Execute method. For now I've modified the code that adds a line length constraint and tomorrow I'll add the logic for the point to point constraint.