Update deprecated QAbstractItemModel::reset() to use {begin,end}ResetModel()…

Authored by dcaliste on Sep 20 2018, 7:55 AM.

Description

Update deprecated QAbstractItemModel::reset() to use {begin,end}ResetModel() instead

Replace QAbstractItemModel::reset() with the more recent {begin,end}ResetModel() calls.
Also remove the deprecated calls to setSupportedDragActions() and replace them by implementing supportedDragActions().

Reviewed By: leinir

Differential Revision: https://phabricator.kde.org/D15102