Do not treat filename in selection as URL

Summary:
KFileWidget::setSelection(QString &) accepts either absolute URLs
or relative paths. If the filename contains a :, it gets treated
as a URL and gets rejected. This forces setSelection to parse
it as URL.

CCBUG: 376365

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4579