Drop support for map format 5.
Don’t crash when saving automatically-upgraded maps.
Introduce map Visibility field.

This replaces the Selectable, UseAsShellmap, and special-cased Type = "Mission" fields.
Allow outside code to specify the map chooser visibility filter.
Upgrade shipped maps to the latest format.
Fix an unrelated editor regression.
Merge pull request #7134 from pchote/map-class

Introduce map Class field to replace Selectable/UseAsShellmap