QFileDialog in Qt 5.9 has better support for mimetype filters and
introduces the QFileDialog::selectedMimeTypeFilter() method [1].
This change implements the required functions on the Plasma side. We
cannot depend on Qt 5.9 yet, so the unit test is compiled only when
building against Qt 5.9.
The manual test is also expanded.
[1]: http://code.qt.io/cgit/qt/qtbase.git/commit/?h=5.9&id=34f82b8abcb279542b6350e70609c549e39caafb