* Remove all existing actions when falling back to using a custom layout to
prevent duplicating those buttons
* Enable clear button of ClearComboBox by default for consistency with
other widgets
Of course it would is possible to add actions directly to the line edit.
These new helper functions have the advantage that they provide a fallback
to the old button overlay implementation in case line edit actions can not
be used.
* Unify/streamline code
* Use Qt::SC_ComboBoxEditField/Qt::SC_SpinBoxEditField to determine the edit
area for better compatibility accorss various styles