[NUI][AT-SPI] Add indexable AccessibilitySuppressedEvents

[NUI][AT-SPI] Add indexable AccessibilitySuppressedEvents

This commit changes the syntax used to modify the collection of
suppressed AT-SPI events (D-Bus signals) to match the indexing syntax
used in DALi:

accessible->GetSuppressedEvents()[AtspiEvent::MOVED_OUT] = true;

view.AccessibilitySuppressedEvents[AccessibilityEvent.MovedOut] = true;