[DONE][Suggestion] Custom briefing for base defense missions

[Finnik]

This feature can enable modders to create diverse lore and background for different types of base defense missions (that are technically already possible with new features like https://openxcom.org/forum/index.php/topic,11429.0.html and https://openxcom.org/forum/index.php/topic,10808.0.html)

Currently, the game uses can spawn alien mission for base defense encounter, that specify the UFO to "land" near the base. Battlescape encounter uses base to generate the map and `alienDeployment.data` section of the landed UFO to generate enemies (race taken from alien mission). But the briefing data is hardcoded and can't be modded.

Possible design - make a new property in `alienDeployment` rule to override vanilla logic and use briefing data from UFO's `alineDeployment.briefing` (currently, it is ignored), like it is done for terror missions.

[Meridian]

Done.

Example:

Code: [Select]

ufos:
  - type: STR_BATTLESHIP
    missionCustomDeploy: STR_MERIDIAN_DEPLOY  # used for deployment data for mission sites and base defense and/or briefing data for base defense

alienDeployments:
  - type: STR_MERIDIAN_DEPLOY
    briefing:
      title: STR_MERIDIAN_TITLE
      desc: STR_MERIDIAN_DESCRIPTION   # if this attribute is not empty, the whole briefing section is used as override
      palette: 0
      background: BACK01.SCR
      music: GMENBASE