Add on-leave toggle for pilots.

Pilots on leave will not be assignable to any flights (but will not be
unassigned from any already scheduled this turn).

https://github.com/dcs-liberation/dcs_liberation/issues/276

qt_ui/windows/AirWingDialog.py

@@ -41,8 +41,9 @@ class SquadronDelegate(TwoColumnRowDelegate):
             return self.squadron(index).nickname
         elif (row, column) == (1, 1):
             squadron = self.squadron(index)
+            active = len(squadron.active_pilots)
             available = len(squadron.available_pilots)
-            return f"{squadron.size} active pilots, {available} available"
+            return f"{squadron.size} pilots, {active} active, {available} unassigned"
         return ""