Implement support for selected mime type filters

Summary:
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

Test Plan:
New unit tests pass when building against Qt >= 5.9.
Manual test also works as expected.

Reviewers: #plasma, dfaure

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5446