Allow deleting waypoints.

In almost every case this leaves us with a flight plan we can't reason
about, so it gets degraded to `CustomFlightPlan`. The exception is when
deleting a target point when there are other target points remaining.
This probably gets people using this feature back to what they want
though, which is essentially the 2.1 behavior.

Fixes https://github.com/Khopa/dcs_liberation/issues/393

qt_ui/windows/mission/flight/waypoints/QFlightWaypointTab.py

@@ -12,8 +12,12 @@ from PySide2.QtWidgets import (
 
 from game import Game
 from gen.ato import Package
-from gen.flights.flight import Flight, FlightType
-from gen.flights.flightplan import FlightPlanBuilder
+from gen.flights.flight import Flight, FlightType, FlightWaypoint
+from gen.flights.flightplan import (
+    CustomFlightPlan,
+    FlightPlanBuilder,
+    StrikeFlightPlan,
+)
 from qt_ui.windows.mission.flight.waypoints.QFlightWaypointList import \
     QFlightWaypointList
 from qt_ui.windows.mission.flight.waypoints\
@@ -103,10 +107,30 @@ class QFlightWaypointTab(QFrame):
     def on_delete_waypoint(self):
         wpt = self.flight_waypoint_list.selectionModel().currentIndex().row()
         if wpt > 0:
-            del self.flight.points[wpt-1]
+            self.delete_waypoint(self.flight.flight_plan.waypoints[wpt])
             self.flight_waypoint_list.update_list()
         self.on_change()
 
+    def delete_waypoint(self, waypoint: FlightWaypoint) -> None:
+        # Need to degrade to a custom flight plan and remove the waypoint.
+        # If the waypoint is a target waypoint and is not the last target
+        # waypoint, we don't need to degrade.
+        flight_plan = self.flight.flight_plan
+        if isinstance(flight_plan, StrikeFlightPlan):
+            if waypoint in flight_plan.targets and len(flight_plan.targets) > 1:
+                flight_plan.targets.remove(waypoint)
+                return
+
+        if not isinstance(flight_plan, CustomFlightPlan):
+            flight_plan = CustomFlightPlan(
+                package=self.flight.package,
+                flight=self.flight,
+                custom_waypoints=flight_plan.waypoints
+            )
+
+        flight_plan.waypoints.remove(waypoint)
+        self.flight.flight_plan = flight_plan
+
     def on_fast_waypoint(self):
         self.subwindow = QPredefinedWaypointSelectionWindow(self.game, self.flight, self.flight_waypoint_list)
         self.subwindow.finished.connect(self.on_change)