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:

After:

Notifications

Before:

After:

Battery monitor

Before:

After:

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