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/PrefsManager.h

@@ -240,6 +240,7 @@ public:
 	Preference<ShowDancingCharacters>		m_ShowDancingCharacters;
 	Preference<bool>	m_bUseUnlockSystem;
 	Preference<float>	m_fGlobalOffsetSeconds;
+	Preference<float>	m_fDefaultGroupOffsetSeconds;
 	Preference<int>	m_iProgressiveLifebar;
 	Preference<int>	m_iProgressiveStageLifebar;
 	Preference<int>	m_iProgressiveNonstopLifebar;