Reset the plot's selection when select is called

Updated the base vtkPlot's virtual method to reset the selection when
select methods are called, giving the expected base behavior for a plot
with no selection capabilities. Without this it can cause weird bugs
when vtkChartXY accumulates the selection from all plot objects for
example.

Change-Id: I42bef78d4c6ca6a4d84e3b0cac34a8757c7075bd
Merge topic 'plot-reset-selection' into master

acf00d72 Reset the plot's selection when select is called