Fix needssave state management
When opening the "Advanced" page we first get onEnabledChanged with false and then with true for the comboboxes.
This causes the KCM to be in modified state despite no changes being made. When closing the KCM this further confuses systemsettings, causing a crash
To avoid this restructure the code so that settings are only written when interactively changing the checkbox value
SENTRY: SYSTEMSETTINGS-2DD