buildingAI: Split building attack and rally point, non-random buildingAI

[real_tabasco_sauce]

Randomly selecting targets for each arrow from a building is not ideal for gameplay's sake: -not very effective vs rushes -mainly more effective en mass (ie turtling)

It is nice for this sort of thing to be player controlled. It is more interesting and exciting this way, and adds playable content.

First, the building arrows should work somewhat like UnitAI, with the closest unit being attacked by default. (​https://code.wildfiregames.com/D4964) Then, buildings should be given the ability to focus their fire, similarly to unitAI, returning to default once the focused unit is deceased or out of range. The controls for attacking vs setting a rally point will need to be separated with a hotkey enabling one over the other. This would probably cause issues with siege towers and ships as long as they use buildingAI.

[Freagarach]

First point should be possible after #5889, that would also imply the second point. See also #2010, and #4000 for Siege and Ships.

Closing in favour of #2010.

[real_tabasco_sauce]

I guess there is an overlap between the two.