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

- move the maxlen attribute to OE_stitch class with a stitcchMaxLen funct (every child class can use it)
- add a defaultLinkMaxLen for the linkstitch class
- remove start and end attribute in linestitch (subcurve class make them obsolete)
- add get functions for curves in OE_controller class

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

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

refresh doxygen documentation

second move to a MVC design