Adjust selection of buildings when having engineers of multiple tech / factions

This issue originates from the [forums](https://forum.faforever.com/topic/2714/the-function-of-automatically-building-mass-storage-around-the-miners/29).

It involves these lines of code in [gamemain.lua](https://github.com/FAForever/fa/blob/deploy/fafdevelop/lua/ui/game/gamemain.lua#L541). The idea is that:
- When having engineers of multiple factions selected, the most common faction / the faction with the highest tech / the faction with the most build power is chosen and other engineers that can not construct these buildings will assist.
- When having engineers of multiple techs selected, the highest tech is chosen and available and other engineers that can not construct buildings will assist.

It is do-able, but tricky:
- How do we take into account other factions (nomads, SCTA).
- How do we queue up structures when some can be build but others can not?
- How do we prevent guard orders from being lost (this happens and is sadly engine related).