DnD and Clipboard support for embedded scenes (e.g. JFXPanel)

GlassViewEventHandler contains a number of DnD related methods:

  - handleDragEnter
  - handleDragExit
  - handleDragOver
  - handleDrop

JFXPanel doesn't sent any of these.

Note that Clipboard support may or may not be borrowed from Glass (as we do for event handling): it is not related to embedding and to any particular scene/stage peer.