workaround apparent vala compiler bug

symtom: allgames list is empty after first platform scan
cause: inside Data.platforms().save_platform(platform, platform.id, ...),
  id parameter has changes to '' after entity save, causing the platform to be
  cached under a blank key until next program run
root-cause: ?? string parameter not actually immutable as expected ??

workaround: use Data.platforms().save_platform(platform, platform.id.dup(), ...)