Add new preference: DefaultGroupOffsetSeconds to dictate the default sync bias on the machine when no group.ini is present (or if there is no SyncOffset field in said file).

- Generally Supports either ITG (9ms) or NULL (0ms).
- Alternatively, one could edit the preference manually to put a different value
- Name/Description probably worth ironing out

src/SongManager.cpp

@@ -418,8 +418,6 @@ void SongManager::LoadSongDir( RString sDir, LoadingWindow *ld, bool onlyAdditio
 				const std::vector<Steps*>& vpSteps = pNewSong->GetAllSteps();
 				for (Steps* s : vpSteps)
 				{
-					// Empty TimingData means it's inherited
-					// from the song and is already changed.
 					if( s->m_Timing.empty() )
 						continue;
 					s->m_Timing.m_fBeat0GroupOffsetInSeconds = group->GetSyncOffset();