Have the wallpaper combo managed by KConfigDialogManager
Summary:
We use an intermediate property which maps to an index since it's
exactly what KConfigDialogManager expects with a QComboBox.
Also relayouted a bit the ctor and the file to make it easier to
understand (several aspect were weaved together).
Reviewers: Plasma, davidedmundson, crossi, bport
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25322