Proxy KCM: Show a warning when a KIO-specific setting is selected
Third party applications and browsers cannot use them. This also includes anything based on QtWebEngine, such as Konqueror with the WebEnginePart and Akregator.
There is no indication that some applications cannot use these settings, which causes user confusion with bugs such as 385292, 398584 and 433575. This change displays a warning message in the KCM if a setting that a non-KIO application cannot use is selected: