Adds OpenEmuHelperApp-Info.plist and embeds it into OpenEmuHelperApp.

This should be useful for issue #375, setting NSSupportsAutomaticGraphicsSwitching for OpenEmuHelperApp.

Note: Xcode won’t automatically relink OpenEmuHelperApp in case its Info.plist file has changed, so make sure to remove OpenEmuHelperApp’s executable file to force plist embedding.