Kicker/Documents optimization: Lazily build KFileItem

Summary:
Save quite a few KFileItem instanciation.
On each document hovering several KFileItem were instanciated for nothing.

Reviewers: #frameworks, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26416