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.

Test Plan:
Before:

After:

Before:

After:

Reviewers: hein, mart, Plasma, ngraham, davidedmundson

Reviewed By: Plasma, ngraham, davidedmundson

Subscribers: abetts, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20257