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.

qwt/examples/bode/main.cpp

@@ -0,0 +1,13 @@
+#include <qapplication.h>
+#include "mainwindow.h"
+
+int main (int argc, char **argv)
+{
+    QApplication a(argc, argv);
+
+    MainWindow w;
+    w.resize(540,400);
+    w.show();
+
+    return a.exec();
+}