Sprite image refactor: Add spritelib

Added spritelib. I haven't tested game patches yet, but there's no way
they're not broken now. This is the final major change to the API,
though. Now all that needs to be done is to add game patch support and
update the game patches again. Eventually location painters will be
added, too.

Also, I've made a few other sprite-image improvements, such as adding a
common SpriteImage_LiquidOrFog class, and eradicating all those weird
sprite image loading functions except LoadBasicSuite (which I shrank,
anyway). Now each loader is tied directly to the relevant sprite
class(es).
Continued maintenance

Game patches still don't work, but now they're closer. I can't look up
something right now because I don't have internet access. Since I had
some extra time (without internet), though, I added Newer midway-flag
images (untested).
Fixes for the Newer midway flag image

Still untested, but I accidentally committed the XL versions of the
midway flag images somehow last time Now they're the right size. I also
added the y-offset value I forgot to add.
Stop sprites.py from importing QtWidgets

There's really no reason for it anymore.
Rename "SimpleDynamic" to "StaticMultiple"

"SpriteImage_SimpleDynamic" -> "SpriteImage_StaticMultiple". That first
one was a temporary name anyway. This also marks the very last planned
change to the Sprite Image API. Gamedefs still don't work, but the
spritelib API is now complete.