Basic FF6 multitap implementation

This patch expands FF6's 2-player support to allow up to 4 players using a multitap.

Changes include:

• the Controller: Single/Multiple config switch is now a Players: 1/2/3/4 slider
• character control in combat is determined by the number of players and the slot the character is in (see below)
• any player can pause at any time in combat, unless the current player has a menu open or is selecting targets
• any player can dismiss end of combat messages (XP, levels gained, etc.)
• any player can control on the field/menu, allowing players to be responsible for their own shopping/character building (think Twitch Plays, but hopefully more civilized!)

Battle controller allocations are as follows:

• For a 2-player setup, character slots 1/3 are controlled by P1, while slots 2/4 are controlled by P2.
• For 3-player, slots 1/2/3 are P1/2/3, and slot 4 also goes to P1
• For 4-player, each controller controls the corresponding character slot

The multitap should be setup on controller port 2 (some emulators call this "5-player" multitap). Remote multiplayer is possible using a service such as Parsec, though I do not personally have any experience with setting this up and can offer no specific advice.