Arc improvements and point to edge constraint

I've imrpoved the arc angle constraint and added an arc panel for the property grid. The property grid now displays the arc radius, start and end angle (in degrees). The arc angle soft constraint is not fully fixed because we still need to project the point that wasn't modified by the solver and that affects the angle value.

I've also worked on implementing the point to edge constraint. For this constraint, a new point is added on the target edge, obtained by projecting the point on the edge. The resulting line (the original point and the projected point) should have a constant length and be perpendicular on the edge in the intersection point for the constraint to be satisfied. For a line this is easy to implement, and for other shapes we need to find the line that is tangent to the edge in the projection point and use that for the perpendicular constraint.