Color line control - affordance suggestion

[pierrelabreche]

The user interface for the color line could be made more intuitive :
- the color line could enable left- or right-click menus adding and removing control points.
- the numbers representing the control points could be displayed with decorations suggesting that they are clickable

[jsachs]

In the next release, hovering the cursor over a tic mark or number changes the shape of the cursor. I also added a right-click menu to add or remove control points.