[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