Waypoints support for structure rally point

[ubitux]

Units have a waypoints support for queuing moves, but structure do not. This would be particularly useful in the early game for let's say, make the engie take a non-usual path, make grens surround the map, or in the late game for air units to avoid anti-air areas.

Note: this may require/involve/relate to the support for shift a-move for units (see #13390)