Updated qwt to 6.3 (qwt-multiaxes branch) (#4497)

Using qwt from https://sourceforge.net/p/qwt/git/ci/qwt-multiaxes/tree/

Applied the following changes to qwt 6.3:
* Added QwtZone to qwt_plot_curve.cpp
* Disabled the emitting of Layout Requests on geometry changes of
  QwtScaleWidget - without this, CPU utilization was up to 100% on
  one core

Fixes #4495

qwt/tests/splineprof/main.cpp

@@ -60,7 +60,7 @@ static void testSplines( int paramType, const QPolygonF& points )
 #endif
 }
 
-int main()
+int main( int, char*[] )
 {
     QPolygonF points;
 

qwt/tests/splinetest/main.cpp

@@ -582,8 +582,10 @@ static void testDuplicates()
     testPaths( "Last point twice", spline, points, points4 );
 }
 
-int main()
+int main( int, char*[] )
 {
     testSplines();
     testDuplicates();
+
+    return 0;
 }