[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
[Platform] Don't show drag-drop dialog unless needed

Don't show drag-drop dialog (move/copy/link)
unless there is actually a choice for the
user (that is, only show it one more than
one of those actions is allowable.)
[Platform] Test drag-drop handler

Test drag-drop handler. Initial test simply
ensures that three options are presented.
[Platform] Add drag-drop handler tests

Add test cases for the drag-drop handler
to satisfy code coverage standards.
[Platform] Add message to drag-drop dialog
[Platform] Consider composition policy before Move

Remove manifestation policy does not ensure
that the user has privileges to modify the
object from which a manifestation is to be
removed, so consult composition policy as
well before enabling the Move variant of
drag and drop.
Merge pull request #259 from VWoeltjen/mct179

[Platform] Add Move, Copy options for Drag-and-Drop