fix(plasmoid): add smallSpacing to leftMargin

Summary: With the other two patches, this aims to make leftMargin consistent in plasmoids. (See https://phabricator.kde.org/D26944 and https://phabricator.kde.org/D26946)

Test Plan:
Before:
{F7977307}
After:
{F7977302}

Reviewers: #vdg, romangg

Reviewed By: romangg

Subscribers: romangg, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26945