Basic implementation of road based transfers.

This adds the models and UIs for creating ground unit transfer orders.
Most of the feature is still missing:

* The AI doesn't do them.
* Transfers can move across the whole map in one turn.
* Transfers between disconnected bases are allowed.
* Transfers are not modeled in the simulation, so they can't be
  interdicted.

https://github.com/Khopa/dcs_liberation/issues/824

qt_ui/widgets/ato.py

@@ -1,7 +1,6 @@
 """Widgets for displaying air tasking orders."""
 import logging
-from contextlib import contextmanager
-from typing import ContextManager, Optional
+from typing import Optional
 
 from PySide2.QtCore import (
     QItemSelectionModel,
@@ -32,11 +31,11 @@ from PySide2.QtWidgets import (
     QVBoxLayout,
 )
 
-from game import db
 from gen.ato import Package
 from gen.flights.flight import Flight
 from gen.flights.traveltime import TotEstimator
 from qt_ui.windows.GameUpdateSignal import GameUpdateSignal
+from ..delegate_helpers import painter_context
 from ..models import AtoModel, GameModel, NullListModel, PackageModel
 
 
@@ -312,15 +311,6 @@ class QFlightPanel(QGroupBox):
         self.flight_list.delete_flight(index)
 
 
-@contextmanager
-def painter_context(painter: QPainter) -> ContextManager[None]:
-    try:
-        painter.save()
-        yield
-    finally:
-        painter.restore()
-
-
 class PackageDelegate(QStyledItemDelegate):
     FONT_SIZE = 12
     HMARGIN = 4