Upgraded Qwt to 6.2 (branch: qwt-multiaxes) (#4427)

This commit is based on https://github.com/GoldenCheetah/GoldenCheetah/pull/3956
with the following additions / changes:
* Upgraded to the latest version of the multiaxes-branch, thus eliminating crashes of GoldenCheetah on startup
* Disabled the emitting of Layout Requests on geometry changes of QwtScaleWidget - without this, CPU utilization was up to 100% on one core
* Added the class SplineLookup, reusing small portions of code from Qwt 6.1
* Re-added the splines in WPrime and RideFile (resampling), using the new interface of QwtSpline
* Appveyor: qwt in cache-section now depends on qwt/qwtconfig.prin.in for refresh on version change

qwt/examples/scatterplot/Plot.h

@@ -0,0 +1,25 @@
+/*****************************************************************************
+ * Qwt Examples - Copyright (C) 2002 Uwe Rathmann
+ * This file may be used under the terms of the 3-clause BSD License
+ *****************************************************************************/
+
+#pragma once
+
+#include <QwtPlot>
+
+class QwtPlotCurve;
+class QwtSymbol;
+
+class Plot : public QwtPlot
+{
+    Q_OBJECT
+
+  public:
+    Plot( QWidget* parent = NULL );
+
+    void setSymbol( QwtSymbol* );
+    void setSamples( const QVector< QPointF >& samples );
+
+  private:
+    QwtPlotCurve* m_curve;
+};