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