Zone creation and edition improvements and corrections :

- It's possible once again to delete points. TODO : Define completely
app behavior when a point is selected.  
- Sliding points extended to middle points. Middle points now appears in
creation mode. Indeed, it's now possible to slide points in edition
mode.
- It's possible once again to add zones in previous ones : a short touch
create a zone, a long select zone.
- User can validate a creation by shortly touching first point, a long
touch selects point, a slide moves.
- "Back" button once again delete the last point in creation mode. TODO
: define behavior or hide in edition mode. If time code a real canceling
system.
- Touching outside the zone in edition mode does nothing. TODO : make
that longer touch, intentional touches make user leaves zone (to save or
not to save, that is the question)  
- Displaying : in creation mode looping projection is now dashed. 
- Lines are now drew with a width depending of the picture size, like
the buttons.
- Zones' edges are now displayed.