flint: Added data index selector

Closes #4450

Provide a select_data_index API for the live scatter plot.

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())

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.