This implements #1364. Based on D1868 by @Freagarach.
An button is added to the menu when units are selected.
If it is pressed, all troops that are gathering resources at the moment will drop the resources at the next dropsite and go to the selected position.
Following units will move:
-Soldiers
-Healers
-Battleships
-Heroes
-Champions
Following units won't move:
-Female Citizens
-Support elephants
-Fishing boats
-Traders
-Units with no attack
Some of those units may not even be able to collect resources. Without excluding some troops, e.g. female citizens or traders would move with the soldiers and this is rarely useful. The selection of units that will move was: Can heal or deal damage.