Win32: Add a command-line (no GUI or ini) option to let the escape button exit the emulator.

This should make PPSSPP a bit more front-end friendly by letting the user simply hit escape to quit the program. The only drawback I could really see is changing UMDs becomes a bit problematic, but I suppose we could make a hotkey for it, if it ends up becoming a problem for front-end users.

This doesn't save to or read from the ini file, since there's no real point currently. The users of such an option would likely be adding other command-line options for PPSSPP to parse, I would think.