Add Lightness Strength option to predefined brushes

Adds a curve widget to the brush options menu to control the strength of the lightness effect on Lightness mode brushes. This allows users to control the impasto effect of these brushes with pen pressure.

Example:

Only issue with it is that the option isn't grayed out when the brushtip is not in Lightness mode. I have not been able to find a way to do that without a massive rewrite of the whole brush options menu system. I do disable the widget, though, so users can't do anything in the curve menu if Lightness mode isn't selected.