Stop swallowing groupby widgets when passing widgets

[jsignell]

This is kind of a follow on to the widget work in #331 and #323 this lets the groupby widget that hvplot creates get through even when other widgets options are used.

import xarray as xr
import hvplot.xarray

import geoviews.tile_sources as gvts
import panel as pn

base_map = pn.widgets.Select(options=list(gvts.tile_sources.keys()), value='EsriImagery')
data = xr.tutorial.load_dataset('air_temperature').air

pn.Column(
    base_map,
    data.hvplot.contour(geo=True, tiles=base_map, widget_type='scrubber', 
                        widget_location='top', widget_layout=pn.Row,
                        levels=10, cmap='reds', line_width=2))

[philippjfr]

Looks great!