Allow maps to be saved during initialisation.
Bump the map format 5 -> 6.

Version 6 ensures that maps define RequiresMod.
Fix map Options serialisation. Fixes #4107.
Move map upgrading to OpenRA.Utility.
Rename utility --minimap to --map-preview.
Upgrade shipped maps to format 6.
Update CHANGELOG.
Merge pull request #4257 from pchote/map-format

Fix map format incompatibilities