OEGameCore: added opt-in for cheat support

Usage: enable -(BOOL)canCheat in core plugin to display Add Cheat menu
in the HUD
Fixes #501.
OEGameCore: added setCheat method for passing cheat code, type and (bool)enabled from UI to core
Fix leaking bound IOSurface texture when rendering a core that supports OpenGL.
Allow cores to set the audio bit depth

32-bit = considered float
Set mBitsPerChannel properly
Also missed setting mFormatFlags properly (only affects theoretical float support)
Zero fill audio if needed
Fix potential crash when failed game load occurs
Fix more misdetection of compressed roms
Import roms dragged into TableView. Fixes #284
Reload visible grid items when zoom level is changed. Fixes #392