Multiplayer: Save game selection inappropriately shared between windows

[ahigerd]

Steps to reproduce:

1. Launch mGBA
2. File -> Save games -> Automatically determine
3. File -> New Multiplayer Window
4. Player 1: Open File -> Save games -- observe that "Automatically determine" is still selected
5. Player 2: Open File -> Save games -> Use player 3 save game

Expected results:
Player 1: Open File -> Save games -- observe that "Automatically determine" is still selected

Actual results:
Player 1: Open File -> Save games -- observe that "Use player 3 save game" is selected

[endrift]

Looking at the way this was implemented...it looks like this was only ever meant to be used in one player mode, and it's (supposed to be) ignored in multiplayer. Which is pretty clearly gonna be very confusing to the user.

[endrift]

This is going to be difficult to fix, and will require some overhauling of the config system so it can actually be different per window. That means I'm gonna have to put this off until 0.11.