PC Engine / Turbografx 16 core

Changes:

• Added mednafen_pce_fast as PCE/TG16 core
• Now you can enter RGUI in-game menu by pressing Select + Start on the controller

Use mGBA as GBA core

Changes:

• Use mGBA as GBA core
• Added PicoDrive as Sega 32X core

Tweaked module release

The following changes were made in this fork:

• Added Stella core for Atari 2600 emulation
• System dir is now '~/system'
• Core info dir is now '~/info'
• RGUI default browser dir is now '/usr/share/games/nes/kachikachi' instead of '/'
• Disabled bilinear filtering for every preset
• Enabled integer scaling
• Greyscale palette for Gambatte core