Add achievement support

[eglaysher]

The Steam edition of Clannad has achievements and it looks like this is done through the DLL.

[AlicesReflexion]

I feel Valve might consider earning achievements/time with another executable "cheating." Dunno how real a risk it is, but it's something to consider. Nobody wants a "I WAS BANNED FOR PLAYING CLANNAD WITH RLVM STAY AWAY!!1!"

[eglaysher]

The game has its own internal achievement tracking which isn't currently working. (Select "Achievements" from the main menu.) That's what this bug is referring to. (The DLL also syncs this data to your Steam profile, and that would be really, really hard to implement for basically no benefit.)

The plan is to just store what achievements were unlocked with the user's save file.

[AlicesReflexion]

Ah. That makes much more sense.

[eglaysher]

When the in game achievement notification shows (not Steam achievement popups), we get:

(SEEN9820)(Line 108):  Undefined: opcode<1:4:457, 1>()
(SEEN9820)(Line 111):  Undefined: WindowLen [opcode<0:3:340, 0>](11)
(SEEN9820)(Line 123):  Undefined: opcode<1:4:456, 1>()

I'll need to implement WindowLen and whatever these two other functions are. Also should dig around in SEEN9820...