Overview and Qt Charts in QT 5.8

.. enforce use of QT 5.8 for building the Overview and
   use of Qt Charts.

.. Qt 5.7.x doesn't neccessarily come with Qt Charts as it
   was a technology preview (and breaks the current Appveyor
   builds)

src/src.pro

@@ -634,8 +634,9 @@ greaterThan(QT_MAJOR_VERSION, 4) {
         SOURCES += Train/BT40Controller.cpp Train/BT40Device.cpp
     }
 
-    # qt charts in QT5.7 or higher
-    greaterThan(QT_MINOR_VERSION, 6) {
+    # qt charts is officially supported from QT5.8 or higher
+    # in 5.7 it is a tech preview and not always available
+    greaterThan(QT_MINOR_VERSION, 7) {
         QT += charts
 
         # Dashboard uses qt charts, so needs at least Qt 5.7