Make sure we always set a default mime filter in save mode

KFileWidget::setMimeFilter() documentation says a default mime filter
should be set if the dialog is being open in save mode.

Without a default filter, the dialog uses a custom item (created by
concatenating all the mime type filters) as default. This is not useful
because it means an additional click for the user to select a filter that can
actually be useful.

If the initiallySelectedMimeTypeFilter() is empty, we use the first mime
filter as fallback.

Test Plan:
tests/qfiledialogtest --mimeTypeFilter application/json --mimeTypeFilter application/zip --acceptMode save

New unit test passes.

Reviewers: Plasma, dfaure

Differential Revision: https://phabricator.kde.org/D5956