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/rawinput/rawinput.h

@@ -171,6 +171,7 @@ namespace rawinput {
 
         void __stdcall devices_print();
         Device *devices_get(const std::string &name, bool updated = false);
+        bool keyboard_combo_pressed(uint16_t first, uint16_t second);
 
         inline std::list<Device> &devices_get() {
             return devices;