Compare Mode Connected Up

.. Connected the compare mode on/off switch in the
   compare pane to Context to notify charts

.. the scope bar and chart painting now highlight in
   red when compare more is active and supported by
   a particular chart.

.. each chart must implement isCompare() to return
   true or false if it is supporting compare mode.

We /still/ do not have a mechanism for clearing or
reordering items in the compare pane, nor a mechanism
for choosing 'delta' compare (i.e. baseline all compares
to first item in the list).