Fix incorrect vertical spacing between main layout and individual wallpaper plugins
Summary: As elaborated in D19873 the vertical spacing between the master form layout and a specific wallpaper plugin was always too big. By setting the main column layout's spacing the 0 we avoid the added vertical space.
Reviewers: hein, mart, Plasma, ngraham, davidedmundson
Reviewed By: Plasma, ngraham, davidedmundson
Subscribers: abetts, plasma-devel
Differential Revision: https://phabricator.kde.org/D20257