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
replace PlayMusicOnMapLoad trait with Lua Media.PlayMusic method
auto play shellmap music (again) on all mods

closes #6071
add Media.StopMusic()
play mission won theme
dispose the result, too
don't shuffle while playing music already
update the music player widget when the music changes
adhere to the music player shuffle and loop checkboxes
fix a NullReferenceException