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/delegate_helpers.py

@@ -0,0 +1,13 @@
+from contextlib import contextmanager
+from typing import ContextManager
+
+from PySide2.QtGui import QPainter
+
+
+@contextmanager
+def painter_context(painter: QPainter) -> ContextManager[None]:
+    try:
+        painter.save()
+        yield
+    finally:
+        painter.restore()