add a new subcurve class (parametric subcurve) and a joincurve (concatenate curves)

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

a linkstitch is created automaticaly in the controler before every new stitch
- add a refreshStitchs function to the controler to refresh all the stitchs of the document

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)

add a maxlen option for the birail stitch (add intermediate point when a stitch is longer than maxlen)

refresh doxygen documentation