bugfix(map): Write and scan the map cache with the platform separator

[bobtista]

• Closes Map cache is not written on non-Windows builds #3101
• Depends on refactor(lib): Add path separator helpers to PathUtil #3105 and bugfix(map): Guard the fallback map display name against a missing separator #3106, whose commits are included here until they merge

writeCacheINI does filepath.concat('\\') before appending the file name, and loadMapsFromMapCacheINI and loadMapsFromDisk build their paths the same way. Off Windows that character lands inside the file name:

before:  <user maps dir>/Maps\MapCache.ini
after:   <user maps dir>/MapCache.ini

Now all three go through getPathInDirectory and getLastPathSeparator, so the cache is written where it is read from and the map path shape is checked against whichever separator the listing actually used.

Todo:

• A client with no existing cache writes maps/MapCache.ini at the correct path
• Replicate to Generals — N/A, it's in Core