Add support for interpolation

It support automaticly moving a frame now. Though some interface should
be added later. Currently it can be done by inserting clone layer (of
previous frame) as a frame by hand and moving it... Intermidiate frames
will have intermidiate position.

Just some bugs: it will not go ok with saving now: you need to delete
all intermidiate frames by hand; also there is not fully ok with
redrawing after changing frame..