BUG: Fix localization and inconsistent state in ctkModalityWidget

There were several issues with the imaging modality selector widget (ctkModalityWidget):
- when imaging modality abbreviations (CT, MRI, XA, ...) were localized then the widget did not work at all, because the checkbox text was used to identify the checkboxes
- widget state was stored redundantly in SelectedModalities and VisibleModalities variables and the checkbox properties; and these were often inconsistent

Fixed by using Qt object names to identify checkboxes; and removing SelectedModalities and VisibleModalities variables (store widget state in the checkbox properties).