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/graphics.h

@@ -134,6 +134,7 @@ void graphics_hook_subscreen_window(HWND hWnd);
 void graphics_screens_register(int screen);
 void graphics_screens_unregister(int screen);
 void graphics_screens_get(std::vector<int> &screens);
+void graphics_poll_screenshot_hotkey();
 void graphics_screenshot_trigger();
 bool graphics_screenshot_consume();
 void graphics_capture_trigger(int screen);