Use native dialog overwrite check

Summary: Past review https://git.reviewboard.kde.org/r/128452/ avoided double overwrite check by disabling the dialog one, it should have been the opposite, reusing QFileDialog.

Reviewers: ngraham, Frameworks, Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D21401