Curve Widget: allow selection of control points of several curves + fix
some bugs in the keyDragMaxMovement.
I wanted to add scale/rotate/shear capabilities but we would also need
to make sure we don't bypass limits when scaling: 1 keyframe on the
right of another must always remain so and that means we have to do
extensive computations on mouseMoveEvent
Fix a bug in the refreshing of the keyframes tangents: they must be
ordered correctly
Merge branch 'workshop' of https://github.com/MrKepzie/Natron into workshop