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/launcher/superexit.h

@@ -2,6 +2,5 @@
 
 namespace superexit {
     bool has_focus();
-    void enable();
-    void disable();
+    void handle_hotkeys(bool alt_f4, bool mapped_exit);
 }