Fixed QwtPlotItem.setItemInterest args

Added missing state default value (boolean=True) to fit the corresponding Qwt C++ method signature:
http://qwt.sourceforge.net/class_qwt_plot_item.html#ab65cbfe489ff73e32a919a0633298fb7

Author: PierreRaybaut

qwt/plot.py

@@ -1834,7 +1834,7 @@ def testItemAttribute(self, attribute):
         """
         return bool(self.__data.attributes & attribute)
 
-    def setItemInterest(self, interest, on):
+    def setItemInterest(self, interest, on=True):
         """
         Toggle an item interest