Fit MGO-related classes into an inheritance tree.

So far, added complexity to no gain.
signs in world 7
minor graphics tweaks?
added pixies, only one phrase per pixie though
Make some use of unified gemgos.

Still a lot of ugly hacks to support unpoked code.
Layered rendering (buggy), World cleanup.
pixie poking
Typo in Player.detonate()
Misc layered-rendering fixes.

Some of these are very ugly.
Remove above-player sprites when tiles destroyed.
Add 'dirtycache' setting to CCDQuest.cfg

Forces regeneration of npy map cache files.
Enabled in current commit.
Trees need a ground sprite to avoid glitches.

Grass is the obvious choice.
Change Fire sprite.
Sprite layer offsets.

All these magic numbers* in images are going to be a problem sometime.
*(Layer offsets and Terrain[] indexes).
Less fragile terrain-sprite offsets, indexing.

Sort order of terrain sprites (on a given level)
is same as definition in images.py.

No more memorising sprite numbers, either.
Fix undrawn tiles across map edges.

Also remove unwanted debug printing.
Refix nonvisible bug on small maps.

Actually remove the unwanted debug printing.
Accursed significant whitespace!
Fix minor glitch with overlapping sprites on cell edges.
Fix utterly-broken 'fix'.

HOW DID I WRITE SOMETHING THAT BROKEN?!