When an interval is added to the compare pane we now
compute the RideFileCache so it can be re-used in the
Histogram and CP charts without computing it each time.
Now allows you to add intervals and select them and
change colors they will use.
It will maintain the data as a CompareInterval which
also includes a RideFile representation of the interval.
This is stored in Context in a QList as compareIntervals
and can therefore be re-used across all charts.
We now need to think about putting some widgets onto the
Compare 'bar' for turning compare mode on and off and
deleting / reordering / clearing items.
