unselectable walls for Red Alert

closes #4544
unselectable walls for Tiberian Dawn
update CHANGELOG
Merge pull request #4558 from Mailaender/unselectable-walls

Unselectable Walls
Added OnNotifyBlockingMove to Mobile
Added a blocking check to Production::CanUseExit
OnNotifyBlockingMove now checks if the unit is idle, and your own or an ally
Small cleanup in Harvester::OnNotifyBlockingMove
Made Move::NotifyBlocker handle traits appropriately