declare dependents objects to their reference (to be able to remove the reference without error)

- draw controls only for the selected curves and stitchs

- fix a bug in birailstitch class (initialisation of len and maxlen variable was not set)
- add the display of a point at 0,0

add thread class (stitch one, not computing) to deal with color, width and later (maybe) rendering style

- bug fix in curves (the subcurve function return bad path in case of start and end both on the same segment)

- correction d'un bug lorsqu'une subcurve etait créee avec startPoint=endPoint (les curves cycliques ne fonctionnaient pas dans tous les cas)

second move to a MVC design