Add play/pause features to the sim.

There's no UI feedback for this yet other than the log messages.

https://github.com/dcs-liberation/dcs_liberation/issues/1704

game/polldebriefingfilethread.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+import json
+import logging
+import os
+import time
+from pathlib import Path
+from threading import Event, Thread
+from typing import Callable, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from game.debriefing import Debriefing
+    from game.sim import MissionSimulation
+
+
+class PollDebriefingFileThread(Thread):
+    """Thread class with a stop() method. The thread itself has to check
+    regularly for the stopped() condition."""
+
+    def __init__(
+        self,
+        callback: Callable[[Debriefing], None],
+        mission_sim: MissionSimulation,
+    ) -> None:
+        super().__init__()
+        self._stop_event = Event()
+        self.callback = callback
+        self.mission_sim = mission_sim
+
+    def stop(self) -> None:
+        self._stop_event.set()
+
+    def stopped(self) -> bool:
+        return self._stop_event.is_set()
+
+    def run(self) -> None:
+        if os.path.isfile("state.json"):
+            last_modified = os.path.getmtime("state.json")
+        else:
+            last_modified = 0
+        while not self.stopped():
+            try:
+                if (
+                    os.path.isfile("state.json")
+                    and os.path.getmtime("state.json") > last_modified
+                ):
+                    self.callback(
+                        self.mission_sim.debrief_current_state(Path("state.json"))
+                    )
+                    break
+            except json.JSONDecodeError:
+                logging.exception(
+                    "Failed to decode state.json. Probably attempted read while DCS "
+                    "was still writing the file. Will retry in 5 seconds."
+                )
+            time.sleep(5)