Interval Compare Pane

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.