Add ability to left-drag group icons

[edelle]

You can't drag the AI squad marker to load all into the truck, workaround is putting the squad leader in by hand, and then highlighting the rest of the fireteam and putting it into a truck.

[jameslkingsley]

Technically this works as intended. You can only left-click-drag individual unit icons. So you could select the group via the group icon, then drag one of the unit icons onto the vehicle and all units in selection will be put into the vehicle.