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/misc/hotkeys.h

@@ -0,0 +1,13 @@
+#pragma once
+
+namespace hotkeys {
+
+    // ALT+F4 monitoring spans boot and teardown; configured actions are enabled separately
+    void start();
+    void enable_raw_input();
+    void enable_input();
+    void disable_input();
+    void disable_raw_input();
+    void stop();
+    bool consume_screenshot();
+}