Add support for uploading .gif as sprite/backdrop and costumes

[carljbowman]

Desired Behavior

From the Upload from Computer option, uploading a .gif would either...

• For a sprite/backdrop, create a new sprite/backdrop and add each frame of the .gif into an individual costume
• For a costume, add each frame of the .gif as costumes within a sprite

Initial UX Considerations

• In the case of adding costumes to an existing sprite, we would need to think through where in the costume list the new costumes are added.

[towerofnix]

This probably closes #3254 by having more information.

[carljbowman]

@towerofnix - Thanks for tracking that down.

[kyleplo]

Not sure if this is possible, but maybe it could also detect the framerate, and add code to do so. For example, a GIF with a 1 second framerate could add this script: