Wish: after loading another sprite sheet, auto-clear the previous sprites preview

Why:
It might be confusing seeing the new sprite sheet in the lefthand panel while in the righthand panel the sliced preview sprites from the previous sheet are still shown.

Repro:
Load and slice a sprite sheet
load another sprite sheet
notice that in the righthand panel the sliced preview sprites from the previously sprite sheet are still shown.

I agree, it's an oversight. Typical of engineering not see this kind of thing, but an experienced qa will, as is happening here. I'll  fix it.