Introduced a new chart selection mode

This commit introduces a new chart selection mode to the charts, based
on plots rather than rows. The default remains row-based selection,
where a selection in one plot causes all corresponding points in the
same row of the table to be selected in other plot objects. This
behavior was improved slightly to iterate through and accumulate
selections in all plots (rather than just using that of the last plot
which resulted in confusing behavior).

The plot based selection keeps any selection specific to a plot. The
vtkPlot object that originated a selection is stored in the PROP
property of the vtkSelectionNode, with the SOURCE property being used to
store the table the selection is on. The vtkSelection object will have a
vtkSelectionNode object for each plot with a selection on it. All of the
normal selection combination rules are applied per-plot.

The rectangle and polygon selection mode code was refactored to reduce
the amount of code duplication necessary. Two tests were introduced
allowing interactive use of the chart with row or plot based selection
using polygon selection (left mouse) or rectangle selection (right
mouse).

Change-Id: Ib739ec620b7094bcf4940767000e8620c432e341