Imported animated PNG files only import the first frame

[rschamp]

Expected Behavior

Please describe what should happen
When you import an animated image as a costume or backdrop, it imports each frame as a separate costume or backdrop (see: importing GIF files)

Actual Behavior

Describe what actually happens
Importing an animated PNG only imports the first frame of the animation

[apple502j]

We use omggif to split GIFs. APNG support probably requires another library with a similar functionality.