[Platform] Add separate handler for drag-drop

Add separate class to handle drag-and-drop.
This will serve as a container for the
copy, link, and move options from
nasa/mct#179.
[Platform] Hook in new drag-drop handler

Defer handling of drag-and-drop behavior in
directory area to dialog-launching handler.
[Platform] Focus on components when dropped

Restores functionality which focuses on dropped
components on drop completion (check for
component equality by component id, not
reference equality.)
[Platform] Consult policy when drag-dropping
[Platform] Add Move option

Add Move option to dialog which appears when
dragging and dropping components in the
directory area.
[Platform] Add Copy option to drag-drop dialog
[Platform] Remove unused method, field
[Platform] Fix component order for move/copy/link
[Platform] Disallow copy-in-place by drag

Disallow Copy when a component is dragged to
its current parent. This prevents dialog
from being popped up when user is reordering.

Additionally, add some clarifying comments.
[Platform] Amend unit test for drap-drop revision

Amend unit test such that preconditions and
expected output are consistent with revised
drag-drop results, as described in
nasa/mct#179