GBA Save: Add support for 128kB SRAM and FlashROM saving (bootlegs)

[metroid-maniac]

Add support for 128kB SRAM saving. Used by some Pokemon and Super Mario Advance 4 bootlegs. GBATA-style SRAM bank switching is also supported for support with SRAM-patched ROMs for older flashcarts. Add support for rewritable ROM using flash commands. Used by "batteryless bootlegs" for saving. ROM overlay is stored in the .sav file instead of SRAM, as these carts do not have batteries to retain SRAM contents persistently. SRAM is still used temporarily during gameplay. Add some toggles to the Qt frontend to configure these parameters. Patches for batteryless ROMs can be found at https://github.com/acocalypso/batteryless-patches , Super Mario Advance 4 is of interest as it also uses 128KB SRAM and will not save properly unless this is correctly emulated.

Relates to issue #2517