Fix buggy selection interaction in 2D charts

This fixes a bug in which sometimes the selection rectangle
would not be drawn. This was caused by the invisible vtkChartWarning
class getting the mouse events instead of the vtkPlotXY class.

Now, vtkChartWarning does not accept any mouse events and they are
properly delivered to vtkPlotXY.

Change-Id: I1a3a0eecfa27f886f0fc909bac95f3f8ad34d3d1
Merge topic '14309-chart-selection-buggy' into next

adc2281 Fix buggy selection interaction in 2D charts