Organize the engine/ directory

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.

Makefile

@@ -21,8 +21,8 @@ data/maps/map_data.o \
 data/pokemon/dex_entries.o \
 data/pokemon/egg_moves.o \
 data/pokemon/evos_attacks.o \
-engine/credits.o \
-engine/events.o \
+engine/title/credits.o \
+engine/overworld/events.o \
 gfx/pics.o \
 gfx/sprites.o \
 lib/mobile/main.o

engine/gfx/color.asm

@@ -1,4 +1,4 @@
-INCLUDE "engine/sgb_layouts.asm"
+INCLUDE "engine/gfx/sgb_layouts.asm"
 
 SHINY_ATK_BIT EQU 5
 SHINY_DEF_VAL EQU 10
@@ -445,7 +445,7 @@ LoadMailPalettes:
 .MailPals:
 INCLUDE "gfx/mail/mail.pal"
 
-INCLUDE "engine/cgb_layouts.asm"
+INCLUDE "engine/gfx/cgb_layouts.asm"
 
 Unreferenced_Function95f0:
 	ld hl, .Palette

engine/gfx/crystal_layouts.asm

@@ -203,7 +203,7 @@ Function49496: ; 49496
 	ret
 ; 494ac
 
-INCLUDE "engine/tileset_palettes.asm"
+INCLUDE "engine/gfx/tileset_palettes.asm"
 
 MG_Mobile_Layout02: ; 49706
 	ld hl, .Palette_49732