Button graphic fixed
Graphics for new flattenpathpiece button
Straights rewritte; New "flatten" button

- Straights are now computed pointwise (like the *Arc classes). This homogenizes appearance across classes and finally allows for correct coloring. Help lines still use the old Straights (now called Straight2).

- New button to flatten PathPieces into the (z=0)-plane.

- Several bugfixes and minor adjustments.