add bring to front/send to back to PfPvPlot

Another rightclick menu for intervals, adding a bring to front and send to
back option but only when viewing the pfPvPlot, additionally the algorithm in
PfPv plot for determining which interval a point is used for has been adjusted
to fully populate every interval curve where appropriate (and incorrect
comments and redundant code have been removed).

IntervalItems now have a display sequence number so when you have
overlapping intervals you can bring to front and send to back on the
PfPv plot. The display sequence could be used on other plots if/when
they distinguish between intervals.

Previously, the coloring of intervals on PfPvPlot was determined solely
by the order they were defined which could be quite confusing.

src/BestIntervalDialog.cpp

@@ -283,13 +283,13 @@ BestIntervalDialog::addClicked()
             QString name = resultsTable->item(i,2)->text();
             const RideFile *ride = mainWindow->currentRide();
 
+            QTreeWidgetItem *allIntervals = mainWindow->mutableIntervalItems();
             QTreeWidgetItem *last =
                 new IntervalItem(ride, name, start, stop,
                                  ride->timeToDistance(start),
-                                 ride->timeToDistance(stop));
-
+                                 ride->timeToDistance(stop),
+                                 allIntervals->childCount()+1);
             // add
-            QTreeWidgetItem *allIntervals = mainWindow->mutableIntervalItems();
             allIntervals->addChild(last);
         }
     }