Add invisibility option to "Color this drawing" option (idea from Lazur)

[cl4cnam]

This is an idea from Lazur.
To make a "draw by numbers" game, it would be useful that the game author can point some parts (the numbers), so that the player could make them (and the palette) invisible.

[cl4cnam]

Maybe, just add an option to push the opacity to 255 when a user click a location (this solution is from Lazur).