Support custom drag actions in model browser

The current default logic for DragSourceListener in GraphExplorerComposite will only attempt to use a hardcoded logic for constructing JSON out of the NodeContexts input Resource/Variable.

Sometimes it is needed to allow completely custom implementation of the DragSourceListener or more specifically the TextTransfer implementation of DragSourceListener.dragSetData.

DndBrowseContext could be augmented to support DragActionContributions.