Add QChart to Python Chart (3f of 5)

Added hover code to line chart, so get legend values on hover
and marker points as you mouse over the series and an x-axis
label at the bottom to show current x value.

Also added Utils::removeDP() to remove decimal places from a
number string e.g. 24.000 becomes 24, 987.3440500 becomes
987.34405. This is a hack to avoid handling decimal places
in the user settings, but will keep as reduces the amount
of "digital ink" regardless.

qwt/src/qwt_text.h

@@ -139,7 +139,7 @@ public:
     //! Layout attributes
     typedef QFlags<LayoutAttribute> LayoutAttributes;
 
-    QwtText( const QString & = QString::null,
+    QwtText( const QString & = QString(),
              TextFormat textFormat = AutoText );
     QwtText( const QwtText & );
     ~QwtText();