Add date-based loadout restriction.

Follow up work:

* Data entry. I plan to do the air-to-air missiles in the near term. I
  covered some variants of the AIM-120, AIM-7, and AIM-9 here, but there
  are variants of those weapons for each mounting rack that need to be
  done still, as well as all the non-US weapons.
* Arbitrary start dates.

https://github.com/Khopa/dcs_liberation/issues/490

qt_ui/windows/settings/QSettingsWindow.py

@@ -248,6 +248,30 @@ class QSettingsWindow(QDialog):
         campaign_layout.setAlignment(Qt.AlignTop)
         self.campaign_management_page.setLayout(campaign_layout)
 
+        general = QGroupBox("General")
+        campaign_layout.addWidget(general)
+
+        general_layout = QGridLayout()
+        general.setLayout(general_layout)
+
+        def set_restict_weapons_by_date(value: bool) -> None:
+            self.game.settings.restrict_weapons_by_date = value
+
+        restrict_weapons = QCheckBox()
+        restrict_weapons.setChecked(self.game.settings.restrict_weapons_by_date)
+        restrict_weapons.toggled.connect(set_restict_weapons_by_date)
+
+        tooltip_text = (
+            "Restricts weapon availability based on the campaign date. Data is "
+            "extremely incomplete so does not affect all weapons."
+        )
+        restrict_weapons.setToolTip(tooltip_text)
+        restrict_weapons_label = QLabel("Restrict weapons by date (WIP)")
+        restrict_weapons_label.setToolTip(tooltip_text)
+
+        general_layout.addWidget(restrict_weapons_label, 0, 0)
+        general_layout.addWidget(restrict_weapons, 0, 1, Qt.AlignRight)
+
         automation = QGroupBox("HQ Automation")
         campaign_layout.addWidget(automation)