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.