[WidgetFactory] Move isLocalFile() check, toLocalFile() to setters

Summary:
Avoid converting the KFileItem to a path for the common case of only
showing the file attributes.
This is the first/preparatory part.

Reviewers: #baloo, astippich, ngraham

Reviewed By: astippich

Tags: #baloo

Differential Revision: https://phabricator.kde.org/D20979