LargeTrackInfoDisplay: add forgoten Destroy() call in CreateScene()'s mask
LargeTrackInfoDisaply: replace an IDisposable cast/call with using{} syntax
Fix (almost) all warnings (update hyena to HEAD too)

Only warning left (to be able to enable WarnAsErrors back) is the version
conflict of glib-sharp 2.12 vs 3.0 whose culprit lies in GConfSharp, I
believe.