Add KFileWidget::setSelectedUrl()

Summary:
It turns out that setSelection() cannot take both relative paths (filenames)
and absolute URLs as input. "a:b" can be both, and URLs for files called "a#b"
were handled wrongly.

CCBUG: 376365

Reviewers: fvogt

Reviewed By: fvogt

Subscribers: fvogt, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4937