Improve moddability of various "loading" maps

[Xymanek]

The maps in question:

• Transition map - by default, soldiers sitting in the skyranger (CIN_Loading_Interior)
• Mission intro - most commonly, skyranger flying in and soldiers jumping down
• Post-mission walkup on avenger's "roof deck" (UIAfterAction)

Existing systems

Mods can easily "attach" their own .umaps to existing mission intros:

X2WOTCCommunityHighlander/X2WOTCCommunityHighlander/Src/XComGame/Classes/XComTacticalMissionManager.uc

Lines 73 to 77 in cd2e225

	struct native AdditionalMissionIntroPackageMapping
	{
	var string OriginalIntroMatineePackage; // original matinee package in the MissionIntroDefinition structure for a mission
	var string AdditionalIntroMatineePackage; // additional matinee package to load when OriginalIntroMatineePackage is loaded
	};

X2WOTCCommunityHighlander/X2WOTCCommunityHighlander/Src/XComGame/Classes/XComTacticalMissionManager.uc

Line 99 in cd2e225

var config array<AdditionalMissionIntroPackageMapping> AdditionalMissionIntroPackages; // for modding, allows packages with intros for new character types to be loaded

X2WOTCCommunityHighlander/X2WOTCCommunityHighlander/Src/XComGame/Classes/X2TacticalGameRuleset.uc

Lines 1414 to 1420 in cd2e225

	foreach MissionManager.AdditionalMissionIntroPackages(AdditionalIntroPackage)
	{
	if (AdditionalIntroPackage.OriginalIntroMatineePackage == MissionIntro.MatineePackage)
	{
	`MAPS.AddStreamingMap(AdditionalIntroPackage.AdditionalIntroMatineePackage, kSoldierSpawn.Location, ObjectiveFacing, false).bForceNoDupe = true;
	}
	}

However, the usability of this system is limited because making use of it requires overriding a character template variable

class X2CharacterTemplate /**/;

var(X2CharacterTemplate) string strIntroMatineeSlotPrefix; // prefix of the matinee groups this character can fill in mission intros

There is no straight-forward system for overriding the other 2

Existing solutions in mods

Directly patching SeqAct_Interps (which is rather hacky): https://github.com/Musashi1584/PrimarySecondaries/blob/94fd404571a16bd868fe7bc2025a24eb324fef3c/PrimarySecondaries/Src/PrimarySecondaries/Classes/ShellMapMatinee.uc

Ideal situation

A delegate or a DLCInfo hook that will provide mods with following:

• Base map (e.g. the transition map can be changed using Allow mods to override the transition map #388)
• Units that will "participate" in the loading screen (the logic is more complicated than XCOMHQ.Squad)

Then the mods will respond with the following:

• Additional maps to load (if any)
• Remote events to trigger per-unit (override)

This way, we can reuse the majority of the approach used already by firaxis (+ the (relative) convenience of the editor), while getting the required flexibility

Considerations

The additional maps will need to be unloaded once they are no longer needed