- add creations function
- change functions in joincurve class to insert new curves anywhere and remove them
- add an action to create and add a subcurve to a joincurve

- create an actionSelection only when there is a selection

- replace the array (curves, stitches, threads,...) by list to be more flexible with delete and moves of elements
- refresh curves only when needed

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

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