Undo after export

[Bart Coppens]

Hmmm, I just noticed something very strange: undo doesn't work anymore after I 
export a file (TIFF and PNG tested). Strangely enough, this doesn't happen 
when saving to the native file format. Also, the png export method didn't 
appear to have any code for changing the undo adapter, and at first sight, a 
backtrace didn't show any other krita functions that could cause it as 
well :-/