Provide Military::addToSquad and possibly revise Military::removeFromSquad

[chdoc]

This functionality is necessary to implement DFHack/scripts#1411 and complicated enough that it makes sense to centralize it.

[ab9rf]

initial notes from static reverse engineering:

• squads consist of histfigs. not units, so to be added to a squad a unit has to have a histfig
• adding a histfig to a squad adds a histfig entity link of type SQUAD between the histfig and the squad
• removing a histfig from a squad removes the SQUAD link and adds a FORMER_SQUAD link
• i have not yet determined whether each time a histfig is removed from a squad, whether it gets a new FORMER_SQUAD link, or if there is only one such link per histfig per squad
• adding a histfig to and removing a histfig from a squad optionally adds history events

[myk002]

fixed by #5363