Qwt 6.0.1 Support

Upgrade to QWT 6.0.1, but still uses a locally patched copy
since support for 8 axes has not been included, despite it
being a relatively simple patch.

Fixes #634.
Fixes #567.

src/TreeMapWindow.cpp

@@ -259,7 +259,7 @@ TreeMapWindow::pointClicked(QwtPlotCurve*curve, int index)
     LTMScaleDraw *lsd = new LTMScaleDraw(settings.start,
                         groupForDate(settings.start.date(), settings.groupBy),
                         settings.groupBy);
-    lsd->dateRange((int)round(curve->x(index)), start, end);
+    lsd->dateRange((int)round(curve->sample(index).x()), start, end);
     ltmPopup->setData(settings, start, end);
     popup->show();
 }