bliss 2.0: flint: Added data index selector

Closes #4450 (closed)

Provide a select_data_index API for the live and custom scatter plot.

This also fixes the interactive selection on scatter. This was probably broken since the viewer refactoring.

Tested with the following

flint()
amesh(sx, 0,0.01,1,sy,0,0.01,1,0.01,diode1)
p = flint().get_live_plot("default-scatter")
print(p.select_data_index())

And

import numpy

# Create the plot
f = flint()
p = f.get_plot("scatter", name="My plot")

# Create the data and setup the plot
y, x = numpy.ogrid[:10, :10]
x, y = numpy.zeros_like(y) + x, numpy.zeros_like(x) + y
x, y = x.flatten(), y.flatten()
v = numpy.sin(numpy.sqrt((x-5)**2 + (y-5)**2))
p.set_data(x, y, v)

# The colormap can be updated
p.set_colormap(lut="red")

print(p.select_data_index())

This have to be backported in BLISS 2.1.

In the master it would be good to try to rework the selection API in order to dispatch it in each plot clients. There is some way to reduce the amount of request/selected API which exists in the general flint API.