Port from KFileDialog to QFileDialog.

Summary:
Tricky because of the embedded widget for save options.
Reused the solution found in kuickshow.

Since QFileDialog implements overwrite checking, kill all
the custom code for doing the same.

Test Plan: Save as (new file), save as (existing file)

Reviewers: broulik, andreagenor, tcanabrava

Reviewed By: tcanabrava

Maniphest Tasks: T11576

Differential Revision: https://phabricator.kde.org/D25482