Title management was duplicated without any kind of link between
vtkPVXYChartView and vtkPVPlotMatrixView

This unify it by creating a few pure abstract method in vtkPVContextView
and storing the Title in vtkPVContextView so it can be formatted
by using specific keywords.

This even enables subclass to reimplement the GetFormattedTitle method
and uses their own Keyword in their Title