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.

engine/overworld/map_objects_2.asm

@@ -0,0 +1,70 @@
+LoadObjectMasks: ; 2454f
+	ld hl, wObjectMasks
+	xor a
+	ld bc, NUM_OBJECTS
+	call ByteFill
+	nop
+	ld bc, wMapObjects
+	ld de, wObjectMasks
+	xor a
+.loop
+	push af
+	push bc
+	push de
+	call GetObjectTimeMask
+	jr c, .next
+	call CheckObjectFlag
+.next
+	pop de
+	ld [de], a
+	inc de
+	pop bc
+	ld hl, OBJECT_LENGTH
+	add hl, bc
+	ld b, h
+	ld c, l
+	pop af
+	inc a
+	cp NUM_OBJECTS
+	jr nz, .loop
+	ret
+
+CheckObjectFlag: ; 2457d (9:457d)
+	ld hl, MAPOBJECT_SPRITE
+	add hl, bc
+	ld a, [hl]
+	and a
+	jr z, .masked
+	ld hl, MAPOBJECT_EVENT_FLAG
+	add hl, bc
+	ld a, [hli]
+	ld e, a
+	ld a, [hl]
+	ld d, a
+	cp -1
+	jr nz, .check
+	ld a, e
+	cp -1
+	jr z, .unmasked
+	jr .masked
+.check
+	ld b, CHECK_FLAG
+	call EventFlagAction
+	ld a, c
+	and a
+	jr nz, .masked
+.unmasked
+	xor a
+	ret
+
+.masked
+	ld a, -1
+	scf
+	ret
+
+GetObjectTimeMask: ; 245a7 (9:45a7)
+	call CheckObjectTime
+	ld a, -1
+	ret c
+	xor a
+	ret