Add locking to some UI actions.

This is by no means complete. The bugs that this solves were already in
6.x, but we'd hidden the speed controls for the sim in that release, and
have always said that anything done after pressing "go" the first time
is undefined behavior. This is the first step on making those mid-sim
actions behave correctly.

UI actions such as creating a new package need to be executed between
ticks of the sim. We can either do this synchronously by blocking the UI
until the tick is done executing, acquiring a lock on the sim, executing
the action, then releasing the lock; or asynchronously by queueing
events and letting the sim execute them when it completes the current
tick (or instantly if the sim is paused).

Anything that comes from the new UI (currently just the map) must be
asynchronous because it goes through the REST API, but for the old UI
it's simpler (and because the lock will only be acquired as quickly as
the user can act, shouldn't slow anything down) to do this
synchronously, since it's difficult to use coroutines in Qt.

https://github.com/dcs-liberation/dcs_liberation/issues/1680

qt_ui/simcontroller.py

@@ -1,6 +1,8 @@
 from __future__ import annotations
 
 import logging
+from collections.abc import Iterator
+from contextlib import contextmanager
 from datetime import datetime, timedelta
 from pathlib import Path
 from typing import Callable, Optional, TYPE_CHECKING
@@ -76,6 +78,11 @@ class SimController(QObject):
             self.started = True
         self.game_loop.set_simulation_speed(simulation_speed)
 
+    @contextmanager
+    def paused_sim(self) -> Iterator[None]:
+        with self.game_loop.paused_sim():
+            yield
+
     def run_to_first_contact(self) -> None:
         self.game_loop.run_to_first_contact()