Reorganize flight planning.

Previously we were trying to make every potential flight plan look
just like a strike mission's flight plan. This led to a lot of special
case behavior in several places that was causing us to misplan TOTs.

I've reorganized this such that there's now an explicit `FlightPlan`
class, and any specialized behavior is handled by the subclasses.

I've also taken the opportunity to alter the behavior of CAS and
front-line CAP missions. These no longer involve the usual formation
waypoints. Instead the CAP will aim to be on station at the time that
the CAS mission reaches its ingress point, and leave at its egress
time. Both flights fly directly to the point with a start time
configured for a rendezvous.

It might be worth adding hold points back to every flight plan just to
ensure that non-formation flights don't end up with a very low speed
enroute to the target if they perform ground ops quicker than
expected.

qt_ui/models.py

@@ -162,7 +162,7 @@ class PackageModel(QAbstractListModel):
         """Returns the flight located at the given index."""
         return self.package.flights[index.row()]
 
-    def update_tot(self, tot: int) -> None:
+    def update_tot(self, tot: datetime.timedelta) -> None:
         self.package.time_over_target = tot
         self.layoutChanged.emit()