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