Implement Apply / Close without Saving for preferences

This hooks up the buttons correctly

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Tie all the pieces together and make Preferences work as intended

Not Apply / Close without Saving / OK work as designed. And things get
correctly stored and reset.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>