Improve handling of supported mimetypes

Move mimetype-handling functions from archive-kerfuffle.cpp to own file
(mimetypes.cpp).

In MainWindow::openArchive() set filter using
QFileDialog::setMimeTypeFilters() instead of using
QFileDialog::setNameFilters(). This fixes sorting of filters in the
"Open Archive" dialog with KF 5.20.0. Frameworkintegration 5.20.0 broke
the old sorting with commit 415ad2ed48356c3065c937813888fa1bd2742789.
Now we get the "All Supported Files" filter automatically.

Differential Revision: D1144