Templatize GraphData

Templatize GraphData

* grapher/GraphData.hxx (GraphDataBase): new superclass for
GraphData. Split time data out as a separate vector.
(GraphData): Rewrite as template.
* grapher/GraphWidget.cxx (on_expose_event): Reflect GraphData
templatization. Handle events with string values.
* grapher/GraphWidget.hxx (GraphWidget): Keep pointers to
GraphDataBase objects instead of GraphData.
* grapher/StapParser.cxx (parseData): new member function
(ioCallback): Handle new discreet event
* grapher/StapParser.hxx (StapParser): keep pointers to GraphDataBase
objects instead of GraphData
* testsuite/systemtap.examples/general/grapher.stp: Display actual key
pressed for keyboard event