Improve canDropMimeData/dropMimeData API

Suggestion:

Change

bool canDropMimeData(const QMimeData *data, Qt::DropAction action, int row, int column, const QModelIndex &parent;) const

to more closely mimic the API of QDropEvent:

Qt::DropAction QAbstractItemModel::canDropMimeData(const QMimeData *data, Qt::DropActions possibleActions, Qt::DropAction proposedAction, int row, int column, const QModelIndex &parent;) const

If you return IgnoreAction, that's like returning false, and the event is
ignored. Otherwise whatever you return goes to QDragMoveEvent::setDropAction.

If they want to ask a different question, they hit a modifier key, and we ask
with a different proposedAction

Generally if you returned true now, you'd return proposedAction in this
interface.

Why:

What you gain is the ability to tell a source that allowed both move and copy, proposing move, that you only accept copy and thus give the user the right cursor feedback.

I want to know all the possibleActions() from the QDragMoveEvent (so I can know Copy is actually allowed) and I want to know which one was proposed (so I can default to following the modifier-keys the user was holding down), and I want to be able to override that (if I support one of the possible actions, but not the currently proposed one).