Workaround Qt 5.9.0 behavior change in QAbstractItemModel

Summary:
Commit 6255cb893d4 in Qt 5.9.0 introduced a regression, now if
supportedDragAction is not overriden it will always return -1 (which is
not even a supported value for the enum).

Reviewers: dfaure, franckarrecot, bensi

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D6360