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/overlay/overlay.h

@@ -212,6 +212,7 @@ namespace overlay {
         bool fps_down = false;
         bool hotkey_toggle = false;
         bool hotkey_toggle_last = false;
+        std::mutex hotkeys_mutex;
 
         // true between new_frame()'s ImGui::NewFrame() and render()'s ImGui::Render(),
         // so render() never runs without a matching NewFrame.
@@ -235,4 +236,5 @@ namespace overlay {
 #endif
     void create_software(HWND hWnd);
     void destroy(HWND hWnd = nullptr);
+    bool global_hotkeys_triggered();
 }