This is an informed attempt at reorganizing the engine/ directory by
creating categorized subdirectories, in order to make it easier to
navigate and find things.

The directories created are as follows:
* engine/game: Contains all "minigames", things like the unown puzzle
and slot machine.
* engine/gfx: Contains all handling of graphics. From loading palettes
to playing animations.
* engine/link: Contains all multiplayer functionality.
* engine/menu: Contains all generic/misc. menus and menu code.
Other, more specialized menus are in their own subdirectories (pokedex,
pokegear, party menu, etc).
* engine/overworld: Contains all handling of the overworld. From loading
and connecting maps to wild encounters and the scripting engine.
* engine/pokegear: In the same vein as engine/pokedex, except it could
use some more splitting up.
* engine/pokemon: Contains everything related to manipulating pokemon
data. From the pokemon storage system to evolution and mail.
* engine/printer: Contains everything related to printing things as well
as the printer communication.
* engine/title: Contains intro sequences, title screens and credits.

Introduce PALRGB_WHITE constant for $7fff

# Conflicts:
#	data/items/descriptions.asm
#	data/sprite_anims/framesets.asm
#	engine/crystal_colors.asm
#	engine/events/kurt.asm
#	engine/events/special.asm
#	engine/events/std_scripts.asm
#	engine/events_3.asm
#	engine/item_effects.asm
#	engine/namingscreen.asm
#	engine/scripting.asm
#	engine/stats_screen.asm
#	engine/trade_animation.asm
#	home/audio.asm
#	main.asm
#	maps/BattleTower1F.asm
#	maps/BattleTowerBattleRoom.asm
#	maps/BurnedTowerB1F.asm
#	maps/ElmsLab.asm
#	maps/GoldenrodDeptStore5F.asm
#	maps/GoldenrodUnderground.asm
#	maps/HallOfFame.asm
#	maps/MahoganyTown.asm
#	maps/ManiasHouse.asm
#	maps/MobileBattleRoom.asm
#	maps/MobileTradeRoomMobile.asm
#	maps/RadioTower2F.asm
#	maps/Route35NationalParkGate.asm
#	maps/Route36NationalParkGate.asm
#	maps/Route39Farmhouse.asm
#	tilesets/palette_maps.asm