Add support for multiple saves in GBA emulator files (Goomba/PocketNES/SMSAdvance)

[euan-forrester]

These emulators can run on the GBA in "bundled" mode, where you select a list of game ROMs on your PC and then a program creates a single executable that runs on the GBA and allows you to select between these games.

The save file for that executable contains save info for all of the games.

Right now, we only support extracting one game's save data from the file, but we could make a dropdown menu to display all of them with their associated game title, similar to how N64/PS1 memory cards are done.

My goal for this site is to just support built in emulators on flash carts, and not get sucked into supporting every emulator ever, so it's not a priority to include this. When run from the flash cart OS directly, an individual save file for each game is created.

But it would be easy to add this ability. If there's enough demand we should consider adding it.