Decide upon setting rally points in mixed selections (units and buildings)

[leper]

(Split from #2189)

Currently when selecting units and buildings giving a command always gives a command to the unit, so you can't set a rally point.

Should orders only go to units (as it is now), or only to buildings, or even to both (unit order and rally point)?

[Josh]

I don't think selecting units and building should even be allowed.

[Imarok]

Orders should go only to units when units and buildings selected.

Whether it should be possible to select buildings and units together is another question, not part of this ticket. (But as there are some hybrids as the Xiongnu moving buildings, that might be quite difficult to resolve)