Reorganise intervals by drag and drop

The interval view now allows the user to drag and drop intervals
up and down in the list.

We could extend the drag and drop of intervals to support analysis
of segments/intervals from multiple rides in the future too.

Fixes #405.

src/AllPlotWindow.cpp

@@ -1011,6 +1011,7 @@ AllPlotWindow::setEndSelection(AllPlot* plot, double xValue, bool newInterval, Q
 
             QTreeWidgetItem *last = new IntervalItem(ride->ride(), name, duration1, duration2, distance1, distance2,
                                         allIntervals->childCount()+1);
+            last->setFlags(Qt::ItemIsSelectable | Qt::ItemIsEnabled | Qt::ItemIsDragEnabled);
             allIntervals->addChild(last);
 
             // select this new interval