graphics: submonitor refresh rate option (#507)

## Description of change
Add an option that forces requested refresh rate value for the second
monitor; i.e., instead of 60Hz allow the user to override the adapter to
be 70Hz.

Useful for people who have a touch screen that can't do 60Hz.

I don't know if this would cause any timing issues in IIDX or SDVX, but
it seems to boot and monitor check is unaffected.

## Testing
Tested TDJ And UFC mode with a submonitor that can only do 61Hz.

src/spice2x/launcher/options.h

@@ -31,6 +31,7 @@ namespace launcher {
             GraphicsForceRefresh,
             FullscreenResolution,
             FullscreenOrientationFlip,
+            FullscreenSubRefreshRate,
             Graphics9On12,
             spice2x_Dx9On12,
             NoLegacy,