Dirty Preference editor pop-ups two dialogs when closing the editor

[lmcbout]

After opening the preferences setting and do a modification (No auto-save), if you try to kill the preferences editor, it will pop-up two dialogs to save the new information. Only one pop-up dialog should be enough when pressing "Don't save" button.

[elaihau]

It happens because both PreferenceContainer and PreferenceEditor are Saveable, and therefore the dialog gets created once on each.

[lmcbout]

@elaihau Even if there is two Preferences (editor_container),
we should either save both or cancel both on the first dialog

[vince-fugnitto]

Closing as the bug is no longer reproducible with the new user-interface:

• Added Preferences UI widget #7105