Prevent UnitMotion from stopping on its own, and rename "MoveCompleted" to "MovementUpdate"
UnitAI is now solely in charge of moving and stopping, making UnitMotion behaviour easier to predict, which will ultimately help with unitAI development. It might temporarily make units more resilient than before however.
UnitMotion also tells UnitAI that it's arrived with "MoveCompleted" messages, but these actually could be wrong - unitAI could decide that we didn't want to stop after all - so change the name for something less misleading.
Differential Revision: https://code.wildfiregames.com/D1886