Changed leftMargins to smallSpacing to be consistent

Summary: With other two patches, this aims to make leftMargin consistent in widgets (See https://phabricator.kde.org/D26945 and https://phabricator.kde.org/D26944)

Test Plan:
System tray

Before:
{F7977382}
After:
{F7977379}

Notifications

Before:
{F7977359}
After:
{F7977357}

Battery monitor

Before:
{F7977362}
After:
{F7977361}

Self reminder: networkmanager is still slightly badly aligned

Reviewers: #vdg, #plasma, ngraham

Reviewed By: #vdg, ngraham

Subscribers: gvgeo, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26946