Don't stop gathering after starting by autocontinue when in a formation.

FinishOrder called SetWaitingOnController, although the order (constructing) had pushed another order (gather).
This is done now by telling we finished the order only when really idle.

This seems to boil down to the question: If we issued an order to a formation, and its members have wandered off (imagine attacking an entity and our members have finished the initial target but attack nearby ones) do we want the whole formation to continue attacking or execute the next order.

Also fixes reforming whilst attacking when an attack order was issued by the player.

Reported by: @Langbart at https://code.wildfiregames.com/D2175#182343
Differential revision: https://code.wildfiregames.com/D4282
Tested by: @Langbart