Clean Garrison-related code.

[Freagarach]

There has been a significant effort in cleaning garrisoning-related code, yet more needs to be done.

As discussed on IRC today, the ticket is created to not forget when the person currently working on it disappears suddenly.

A list of possible things:

• Ungarrison using cmpGarrisonable instead on cmpGarrisonHolder (refs. ​Phab:D3280). ​Phab:D3628
• Don't listen to OnGlobalEntityRenamed messages when the only use case is Skirmish Replacements (one can use a new message for those). ​Phab:D3627

[Freagarach]

Again discussed today on IRC.

• UnitAI shouldn't probably care about garrisoning.
• Garrisonable sets UnitAI IDLE.
• Orders/States in UnitAI just check for being able to move.

[Freagarach]

In 25019:

Ungarrison entities using cmpGarrisonable.

Follow-up to r24977 / rP24977.
This moves responsibilities even further to the garrisoning entity.

Fixes the garrison flag when renaming entities cannot garrison (#5906).
Allows easy fixing of #6087.

Ticket: #6081
Differential revision: D3628
Comments by: @Stan, @wraitii (also in length on IRC)