Feature: Ability to hide but not delete same frames

[linojon]

When developing a mask, the goal is to make the mask work on as many frames as possible while keeping each match unique. Once I have a mask, I delete the same frames on a frame to avoid cluttering my strip and move on to the next. But then if i continue to refine the mask on other frames, i dont know if this will break ones that i've already done. It would be helpful to be able to "restore" them so i can double check it. This might be achieved with the ability to hide the set of same frames rather than delete them and then un-hide them.

[zesinger]

Sorry, that would change too many things in the UI, I won't do this one