This is a pretty janky system until we get add context menu support. For
now the destination is set by dragging the CV marker and cleared by
right clicking the destination marker. Once we have a context menu a
context action will begin setting the destination the way it did in the
old UI, and the destination marker will be draggable.
These are an implementation quirk, and passing them to the UI just means
that we put TGO pins on top of the CP, which makes the base menu
unopenable.
In the old UI we avoided this by not drawing anything that was
`for_airbase`, but now that we can zoom in further we're drawing base
defenses.
https://bugreports.qt.io/browse/PYSIDE-1426
For whatever reason this only shows up in packaged builds for us, and
also the recommended workaround of using a member property rather than a
decorated method does not work for us.
Until PySide2 5.15.3 (or later) is released, we need to use a named
signal for every property we expose.
