[DONE] Configuration for UI window backgrounds

[Meridian]

Added also alternative background image for windows that can be displayed both in geoscape and battlescape, e.g.
- soldier armor UI
- soldier avatar UI
- select music UI
- abandon game UI
- notes UI
- save/load UI
- etc.

Example:

Code: [Select]

interfaces:
  - type: soldierArmor
    backgroundImage: BACK14.SCR     # geoscape background
    altBackgroundImage: TAC00.SCR   # battlescape background