SettingsDialog: Do the existing instance check in __new__ instead of __init__ so...

SettingsDialog: Do the existing instance check in __new__ instead of __init__ so that the object isn't even constructed if there is already an instance.
Previously, __del__ was being called even if __init__ raised an exception, so _hasInstance was being cleared when it shouldn't.
Fixes the issue where you could open a second NVDA settings dialog if you tried twice.