new animation key behaviour

[Fazekas László]

Probably this supposed to be a feature request, but I think I can make 
it easily myself if you accept it.

When clicking on the "New Frame" button on the animation dialog, nothing 
happens if there is already a key on the frame. Otherwise it creates an 
empty frame. One of my friends who tried Krita told me that she expected 
a clearing (filling with transparency) of the keyframe in this case. I 
think it's a good idea.

Fazek