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

game/transfers.py

@@ -0,0 +1,76 @@
+import logging
+from dataclasses import dataclass
+from typing import Dict, List, Type
+
+from dcs.unittype import VehicleType
+from game.theater import ControlPoint
+
+
+@dataclass
+class TransferOrder:
+    """The base type of all transfer orders.
+
+    A transfer order can transfer multiple units of multiple types.
+    """
+
+    #: The location the units are transferring from.
+    origin: ControlPoint
+
+    #: The location the units are transferring to.
+    destination: ControlPoint
+
+    #: True if the transfer order belongs to the player.
+    player: bool
+
+
+@dataclass
+class RoadTransferOrder(TransferOrder):
+    """A transfer order that moves units by road."""
+
+    #: The units being transferred.
+    units: Dict[Type[VehicleType], int]
+
+
+class PendingTransfers:
+    def __init__(self) -> None:
+        self.pending_transfers: List[RoadTransferOrder] = []
+
+    @property
+    def pending_transfer_count(self) -> int:
+        return len(self.pending_transfers)
+
+    def transfer_at_index(self, index: int) -> RoadTransferOrder:
+        return self.pending_transfers[index]
+
+    def new_transfer(self, transfer: RoadTransferOrder) -> None:
+        transfer.origin.base.commit_losses(transfer.units)
+        self.pending_transfers.append(transfer)
+
+    def cancel_transfer(self, transfer: RoadTransferOrder) -> None:
+        self.pending_transfers.remove(transfer)
+        transfer.origin.base.commision_units(transfer.units)
+
+    def complete_transfers(self) -> None:
+        for transfer in self.pending_transfers:
+            self.complete_transfer(transfer)
+        self.pending_transfers.clear()
+
+    @staticmethod
+    def complete_transfer(transfer: RoadTransferOrder) -> None:
+        if transfer.player == transfer.destination.captured:
+            logging.info(
+                f"Units transferred from {transfer.origin.name} to "
+                f"{transfer.destination.name}"
+            )
+            transfer.destination.base.commision_units(transfer.units)
+        elif transfer.player == transfer.origin.captured:
+            logging.info(
+                f"{transfer.destination.name} was captured. Transferring units are "
+                f"returning to {transfer.origin.name}"
+            )
+            transfer.origin.base.commision_units(transfer.units)
+        else:
+            logging.info(
+                f"Both {transfer.origin.name} and {transfer.destination.name} were "
+                "captured. Units were surrounded and captured during transfer."
+            )