Instead of directly selecting the view that the panel
list will have, let it decide which is the previous
view instead.
This does not change anything functionality wise, but
in the future where we have sidebar widget (and thus
the main window does not control which view the sidebar
is display) this will be important.
This field can be used to communicate the visibility of the
panel from a static context (i.e. without any instances of
a CcPanel nor any access to CcShell).
