Add queued / multiple orders to rally points

[historic_bruno]

Units support queued orders (waypoints) which the player can use by holding down the shift key when assigning orders. So our rally points should also support queued orders, with the same hotkey. For instance if you want trained units to follow a specific path, you could give multiple rally points with the same shift hotkey. I guess this would require changes to the rally point line renderer to support connecting multiple way points (maybe it should share its implementation with #657) and instead of having a single rally point order, there would be an array with all orders after the first being queued.

[leper]

In 11909:

Queued orders for rally points. Fixes #1053 and #1211.