Flint: Custom plot grid

This PR provides a custom widget for parenting sub plots.

This can be used to structure custom plot together in a single view.

f = flint()
p = f.get_plot("grid", name="Grid", selected=True)
p1 = p.get_plot("curve", row=0, col=0)
p2 = p.get_plot("curve", row=0, col=1)
p3 = p.get_plot("curve", row=1, col=0)
p4 = p.get_plot("curve", row=1, col=1)

p1.add_curve([0, 1, 2, 3], [1, 2, 1, 2])
p2.add_curve([0, 1, 2, 3], [2, 1, 2, 1])
p3.add_curve([0, 1, 2, 3], [3, 2, 1, 0])
p4.add_curve([0, 1, 2, 3], [0, 1, 2, 3])