MimeTypeSettings: Use QTreeView instead of QTableView

QTreeView is a better list view with columns than QTableView.
One special reason for this change is that QTreeView
finds a proper default row height regardless of the system
DPI.

Change-Id: I0a4091585d4745fd91179c82a55682f88f8d2549
Reviewed-by: Eike Ziller <eike.ziller@digia.com>