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:
{F6754037}

After:
{F6754036}

Before:
{F6754043}

After:
{F6754044}

Reviewers: hein, mart, #plasma, ngraham, davidedmundson

Reviewed By: #plasma, ngraham, davidedmundson

Subscribers: abetts, plasma-devel

Tags: #plasma

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