Fix hover for overlays of gv.Points

[hoxbro]

Fixes #618 (the hover problem)

The main problem is the self.handles.get('hover') was the same for all plots in the overlay, where we wanted the hover per plot.

The example given in the issue could be cut down to this MRE:

import geoviews as gv
import pandas as pd

gv.extension("bokeh")
gv.opts.defaults(
    gv.opts.NdOverlay(tools=["hover"], legend_position="right", width=600),
    gv.opts.Points(tools=["hover"], size=10),
)


df = pd.DataFrame(
    dict(
        latitude=range(55, 60),
        longitude=range(-160, -155),
        time=pd.date_range("2020-01-01", freq="M", periods=5),
    )
)

ds = gv.Dataset(df, kdims=["longitude", "latitude", "time"])
ds.to(gv.Points).overlay()

(Note the NaN problem will be fixed in #625)