Select formations as a whole by default.
One can override this behaviour by using a hotkey when (de)selecting entities.
The aim of this system is to reduce micromanagement a bit.
Differential revision: https://code.wildfiregames.com/D2175
Comments by: @Angen, @azayrahmad, @Langbart, @marder, @Stan, @submariner, @wowgetoffyourcellphone, @wraitii
Refs. #4545