overlay: create a dedicated thread for polling hotkeys (#864)

## Link to GitHub Issue or related Pull Request, if one exists
n/a

## Description of change
Create a new thread that polls for the following hotkeys:

* super exit (both alt+f4 and bound key)
* coin insert
* screenshot

The goal is to make the capture of these more reliable, because before
this PR it wasn't.

## Testing

src/spice2x/hooks/graphics/backends/d3d9/d3d9_backend.cpp

@@ -1522,7 +1522,7 @@ void graphics_d3d9_on_present(
         wintouchemu::update();
     }
 
-    graphics_d3d9_poll_screenshot_hotkey();
+    graphics_poll_screenshot_hotkey();
     graphics_d3d9_process_screenshot_and_capture(device, SUB_SWAP_CHAIN);
 }