kcms/tabbox: Handle config directly within the configuration form

By passing the config object to the TabBoxConfigurationForm, we can handle it directly instead of bouncing information from and back between the forms and the KCM main class.

Now the user changes in the UI translate to the config objects, which can directly provide isSaveNeeded() and isDefaults() info.

This allows to simplify the logic on both classes, reducing the coupling between them and minimizing the number of exposed public methods and some boilerplate.