Added 'default_skin' directive to truck file format

[ohlidalp]

Syntax: default_skin Example: default_skin Viper_Yellow/Black_Stripe

This works with classic .skin files (which may be in the mod ZIP itself or separate SKINZIP). When used, the game will display that skin on top of skin selector, instead of the dummy "Default skin" entry. If no other skins are found, the game will apply this skin automatically and won't display the selector at all.

Closes #2990 Closes #2909

Cache file version was bumped from 11 to 12 because it must remember the skin name. The selector UI displays the default skin in the details.

[CuriousMike56]

An issue with the AI menu:

• Select a vehicle with skins (with or without default_skin directive)
• Try to select the same vehicle again or a different vehicle that also has skins. This time the skin selector will be skipped.