Gambatte: added custom palette support

Looks in /OpenEmu/BIOS/palettes/ for rom_name.pal,
internal_rom_header_game_name.pal or default.pal formatted for example:

Background0=11846369
Background1=15851184
Background2=9917014
Background3=0
Sprite%2010=0
Sprite%2011=16316664
Sprite%2012=14778518
Sprite%2013=8337480
Sprite%2020=0
Sprite%2021=16316664
Sprite%2022=14778518
Sprite%2023=8337480