If ctkMessageBox is subclassed and anything is added to the layout under the message text, then the Don't show again button appears incorrectly. The reason is that the checkbox is added to the second row and second column of the main grid layout.

This came up when don't show again checkbox was added to the DICOM references dialog: https://github.com/Slicer/Slicer/pull/767
It was decided to keep subclassing QMessageBox, and implementing the don't show feature. If this comes up again, then ctkMessageBox might need to be changed.

Issues identified with ctkMessageBox:

• setDontShowAgainVisible adds the checkbox in a fixed layout position
• A new OK button is also added when calling setDontShowAgainVisible (this could be a potential source of regressions, as this logic might be relied on in other projects)
  • The new button is added in ctkMessageBoxPrivate::button, which is supposed to be a getter function. Adding a button in this function is unexpected
    Potential solution:
• Add a 0px high and empty QFrame in ctkMessageBox between the message and the don't show button. Any extra content could then be added to this empty frame