This commit solves a bug in kopete nowlistening preferences kcm

To reproduce the bug in nowlistening preferences kcm:

- open the preferences for nowlistening plugin
- in the third tab check the checkbox and choose a mediaplayer
- close the dialog
- open preferences again (the checkbox is now already checked)
- choose another mediaplayer
- close the dialog
==> the mediaplayer-selection is not applied
test that by opening the preferences again and look

This patch replaces the k3listbox by a klistwidget, which solves the
problem, and seems a good idea anyway because k3listbox will be removed
someday.