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/CMakeLists.txt

@@ -578,6 +578,7 @@ set(SOURCE_FILES ${SOURCE_FILES}
         misc/device.cpp
         misc/eamuse.cpp
         misc/extdev.cpp
+        misc/hotkeys.cpp
         misc/sciunit.cpp
         misc/sde.cpp
         misc/wintouchemu.cpp