[Scenario] Remove unused imports
[Scenario] Move Repository impl to shared superclass

Moves bulk of logic/behavior associated with Tag and
Activity Type repositories to a common superclass.
This should simplify subsequent implementation of
move-instead-of-copy behavior when dragging into
repositories.
[Scenario] Class-level javadoc for repository components
[Scenario] Remove components from other repositories on add

When a component is added to one repository (such as
via drag and drop), remove it from all similar repositories.
This ensures that a component exists in no more than
one repository at any given time.
[Scenario] Move components between repositories on SwingWorker

Use a SwingWorker to handle the move of components between
repositories in the background. Additionally, move this
logic out of RepositoryComponent in order to simplify parts.
This also provides a place to display the dialog associated
with this move, which is also specified as part of
nasa/MCT-Plugins#135.
[Scenario] Stub in repository move warning dialog
[Scenario] Add progress bar to repository move dialog
[Scenario] Fix bug in removal portion of repository move

Moving multiple components between repositories would
sometimes fail because getReferencingComponents may
return new versions of parent objects, which were
being used as keys to a map in the removal procedure.
Changing to track parents by ids, and maintaining the
most recently-returned version of each parent.

Additionally, clarify some variable names and fix
some logic in progress reporting.
[Scenario] Disable OK button during repository move

It is important to block user interaction while
the repository move is being completed, in case this
is over a higher-latency connection (calls to
getReferencingComponents consult the database, so
it may take time to complete the operation), in
order to avoid presenting the user an intermediary
state.

Additionally, replace the progress dialog with a
notification (currently a placeholder, will contain
information about which components have been moved
from or to which repositories)