Add
- "restore default value" (for example by right click)
- possibility to change values by typing e.g. into a text entry
After a little search I think the code to change is maybe found at:
https://invent.kde.org/kde/kdenlive/blob/master/data/effects/movit_lift_gamma_gain.xml
https://invent.kde.org/kde/kdenlive/blob/master/data/effects/lift_gamma_gain.xml
https://invent.kde.org/kde/kdenlive/blob/master/src/effectstack/widgets/lumaliftgain.cpp