separate SongManager::Cleanup and SongManager::FlushCaches

Cleanup() is called every screen, to remove large decompressed step
data, but that shouldn't clear trail caches as well;
SongManager::FlushCaches also clears the StepsID cache

stepmania/src/GameState.cpp

@@ -152,7 +152,7 @@ void GameState::Reset()
 
 	/* We may have cached trails from before everything was loaded (eg. from before
 	 * SongManager::UpdateBest could be called).  Erase the cache. */
-	SONGMAN->RegenRandomTrailEntries();
+	SONGMAN->FlushCaches();
 
 	g_vPlayedStageStats.clear();