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/variables.asm

@@ -0,0 +1,151 @@
+_GetVarAction:: ; 80648 (20:4648)
+	ld a, c
+	cp NUM_VARS
+	jr c, .valid
+	xor a
+.valid
+	ld c, a
+	ld b, 0
+	ld hl, .VarActionTable
+	add hl, bc
+	add hl, bc
+	add hl, bc
+	ld e, [hl]
+	inc hl
+	ld d, [hl]
+	inc hl
+	ld b, [hl]
+	ld a, b
+	and RETVAR_EXECUTE
+	jr nz, .call
+	ld a, b
+	and RETVAR_ADDR_DE
+	ret nz
+	ld a, [de]
+	jr .loadstringbuffer2
+
+.call
+	call _de_
+	ret
+
+.loadstringbuffer2 ; 8066c (20:466c)
+	ld de, wStringBuffer2
+	ld [de], a
+	ret
+; 80671 (20:4671)
+
+.VarActionTable: ; 80671
+; entries correspond to VAR_* constants
+	; RETVAR_STRBUF2: copy [de] to wStringBuffer2
+	; RETVAR_ADDR_DE: return address in de
+	; RETVAR_EXECUTE: call function
+	dwb wStringBuffer2,                 RETVAR_STRBUF2
+	dwb wPartyCount,                    RETVAR_STRBUF2
+	dwb .BattleResult,                  RETVAR_EXECUTE
+	dwb wBattleType,                    RETVAR_ADDR_DE
+	dwb wTimeOfDay,                     RETVAR_STRBUF2
+	dwb .CountCaughtMons,               RETVAR_EXECUTE
+	dwb .CountSeenMons,                 RETVAR_EXECUTE
+	dwb .CountBadges,                   RETVAR_EXECUTE
+	dwb wPlayerState,                   RETVAR_ADDR_DE
+	dwb .PlayerFacing,                  RETVAR_EXECUTE
+	dwb hHours,                         RETVAR_STRBUF2
+	dwb .DayOfWeek,                     RETVAR_EXECUTE
+	dwb wMapGroup,                      RETVAR_STRBUF2
+	dwb wMapNumber,                     RETVAR_STRBUF2
+	dwb .UnownCaught,                   RETVAR_EXECUTE
+	dwb wEnvironment,                   RETVAR_STRBUF2
+	dwb .BoxFreeSpace,                  RETVAR_EXECUTE
+	dwb wBugContestMinsRemaining,       RETVAR_STRBUF2
+	dwb wXCoord,                        RETVAR_STRBUF2
+	dwb wYCoord,                        RETVAR_STRBUF2
+	dwb wSpecialPhoneCallID,            RETVAR_STRBUF2
+	dwb wNrOfBeatenBattleTowerTrainers, RETVAR_STRBUF2
+	dwb wKurtApricornQuantity,          RETVAR_STRBUF2
+	dwb wCurrentCaller,                 RETVAR_ADDR_DE
+	dwb wBlueCardBalance,               RETVAR_ADDR_DE
+	dwb wBuenasPassword,                RETVAR_ADDR_DE
+	dwb wKenjiBreakTimer,               RETVAR_STRBUF2
+	dwb NULL,                           RETVAR_STRBUF2
+; 806c5
+
+.CountCaughtMons: ; 806c5
+; Caught mons.
+	ld hl, wPokedexCaught
+	ld b, wEndPokedexCaught - wPokedexCaught
+	call CountSetBits
+	ld a, [wd265]
+	jp .loadstringbuffer2
+; 806d3
+
+.CountSeenMons: ; 806d3
+; Seen mons.
+	ld hl, wPokedexSeen
+	ld b, wEndPokedexSeen - wPokedexSeen
+	call CountSetBits
+	ld a, [wd265]
+	jp .loadstringbuffer2
+; 806e1
+
+.CountBadges: ; 806e1
+; Number of owned badges.
+	ld hl, wBadges
+	ld b, 2
+	call CountSetBits
+	ld a, [wd265]
+	jp .loadstringbuffer2
+; 806ef
+
+.PlayerFacing: ; 806ef
+; The direction the player is facing.
+	ld a, [wPlayerDirection]
+	and $c
+	rrca
+	rrca
+	jp .loadstringbuffer2
+; 806f9
+
+.DayOfWeek: ; 806f9
+; The day of the week.
+	call GetWeekday
+	jp .loadstringbuffer2
+; 806ff
+
+.UnownCaught: ; 806ff
+; Number of unique Unown caught.
+	call .count
+	ld a, b
+	jp .loadstringbuffer2
+
+.count
+	ld hl, wUnownDex
+	ld b, 0
+.loop
+	ld a, [hli]
+	and a
+	ret z
+	inc b
+	ld a, b
+	cp NUM_UNOWN
+	jr c, .loop
+	ret
+; 80715
+
+.BoxFreeSpace: ; 80715
+; Remaining slots in the current box.
+	ld a, BANK(sBoxCount)
+	call GetSRAMBank
+	ld hl, sBoxCount
+	ld a, MONS_PER_BOX
+	sub [hl]
+	ld b, a
+	call CloseSRAM
+	ld a, b
+	jp .loadstringbuffer2
+; 80728
+
+.BattleResult: ; 80728
+	ld a, [wBattleResult]
+	and $3f
+	jp .loadstringbuffer2
+; 80730