Removed the notion of Sprite Sheets vs single frame images

Instead of making a distinction between single frame images and Sprite
Sheets, I added a simple FrameData with one Frame to every image that
gets loaded. This way, a lot of the engine code can be simplified.

For instance, you can play animations on any Sprite without having to
check the “frame count” of its image anymore.

The engine doesn’t have to check for non-existing .frameData anymore.

An animation can keep on playing when an image happens to be set with 1
frame, continuing its animation when after that, another image is set
with multiple frames.

Think of it this way: an image with one frame is an animation of just
one frame.
Removed some more checks
Merge pull request #1059 from lucbloom/spritesheet-to-framecount

Sprite Sheet => frame count