update the wallpaper dialog to the new design

Summary:
update the wallpaper dialog to the same design as the new KCMS
for consistency

Test Plan:
dialog loads with proper wallpaper selected, and both clicking on one
works and the attached toolbuttons work too

Reviewers: Plasma, VDG, davidedmundson, broulik

Reviewed By: Plasma, broulik

Subscribers: davidedmundson, broulik, abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12148