Set IconItem default to smooth=true

Summary:
This changes between linear and nearest neighbour filtering.

Docs for QtQuick say:
In Qt Quick 2, this property has minimal impact on performance.

Generally we round to icon size anyway and resize the underlying images
anyway, so this will have no impact.

Reviewers: Plasma, apol

Reviewed By: apol

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10653