[Canvas] Provide getter for housed view in panel inspector

Overrides getHousedViewManifestation in PanelInspector
such that it returns the currently housed view. This
is used by the view switcher (and potentially other
users); its absence causes nasa/mct#200.
[Canvas] Test getter in PanelInspector
Merge pull request #215 from VWoeltjen/mct200

[Canvas] Provide information needed by View Switcher
[Delete] Copy Delete action to Delete All

Create a copy of the Delete action to serve
as the basis for Delete All, for nasa/mct#89
[Delete] Register delete all with action manager
[Delete] Include 'delete all' in objects menu
[Delete] Specify name of 'Delete All' action

Delete All is specified as the name for the recursive
delete action requested by nasa/mct#89.

This name is chosen for symmetry with the 'Save All'
action introduced in
770df79fdb732f8d60e013623dc1c85904c6ec50
[Delete] Add 'delete all' strings to bundle
[Delete] Algorithms for 'delete all' descendant categories

Place components into two buckets: Components recursively
deleted (removed entirely) and objects which cannot be
deleted, but which exist elsewhere and so will not prevent
this action from occurring.
[Delete] Enable delete all behavior

Invoke the descendant probing algorithms to
assemble lists of components which can be
deleted, which cannot be deleted but would
be removed, and cannot be removed, and
complete the action if it can be completed
without violating the latter.

Dialogs pop up conditionally, but do not
present complete information desired.
[Delete] Show appropriate 'delete all' dialogs
[Delete] Move recursion in 'delete all'

Only recursively visit components of children
which will be deleted; do not propagate the
delete all action past components which cannot
be deleted.
[Delete] Allow custom message objects in dialogs

Support (by way of hints) injecting custom message
components (Swing components) into input/message
dialogs shown through the WindowManagerImpl.

This is useful for delete all specifically, as its
warning messages may contain lists of components
which are going to be deleted (for example)
[Delete] Use Window Manager for Delete All dialog

Use the Window Manager (as opposed to static methods
of OptionBox) to show the warning dialogs related
to the Delete All action introduced for nasa/mct#89.

This slightly reduces the explicit Swing
dependencies associated with the action and
facilitates automated testing of interactions
(the WindowManager can be mocked, but the static
methods of OptionBox cannot). These are ongoing
concerns of nasa/mct#155.
[Delete] Hoist platform references in Delete All
[Delete] Add class comments to DeleteAllAction
[Delete] Skeleton for Delete All test
[Delete] Test canHandle in Delete All
[Delete] Get PolicyManager from platform in Delete All

Instead of using PolicyManagerImpl directly, retrieve
the PolicyManager via the platform. This reduces
dependency (Delete All no longer requires a specific
implementation of policy manager) and supports
unit testing.
[Delete] Test isEnabled in Delete All