Don't ignore initially selected mime type filter

Otherwise QFileDialog::selectMimeTypeFilter() has no effect
if called before show()/initializeDialog() (because the mime
filters are set only in initializeDialog()).

We also need to ignore the initiallySelectedNameFilter() (which is
implicitly set) if we have an initiallySelectedMimeTypeFilter().

Test Plan:
tests/qfiledialogtest --mimeTypeFilter application/json --mimeTypeFilter application/zip --selectMimeTypeFilter application/zip

Moved the show() call in the existing unit test, now it passes.

Reviewers: Plasma, dfaure

Differential Revision: https://phabricator.kde.org/D5957