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/SmallPlot.cpp

@@ -29,7 +29,8 @@
 #include <qwt_plot_marker.h>
 #include <qwt_text.h>
 #include <qwt_legend.h>
-#include <qwt_data.h>
+#include <qwt_series_data.h>
+#include <qwt_compat.h>
 
 static double inline max(double a, double b) { if (a > b) return a; else return b; }