SCheckBox::SetIsChecked does not broadcast OnCheckStateChanged

The implementation for SCheckBox::SetIsChecked simply sets IsCheckboxChecked without checking to see if the passed in value is different and does not trigger OnCheckStateChanged.

1. Add a listener to OnCheckStateChanged for any SCheckBox
2. Call SetIsChecked on that SCheckBox

Result: The added listener will not be called.

Expected result: The added listener should be called.