Allow Tabulator.row_content to be an awaitable

[maximlt]

This PR allows row_content to be an awaitable. When it is an awaitable, we display first a small spacer with a loading spinner, replaced by the content returned by the function.

Micro app to play around with this feature:

import asyncio

import panel as pn
import pandas as pd

df = pd.DataFrame({"value": [0, 1, 2]})

async def async_row_content_cb(event):
    await asyncio.sleep(1)
    print("Event", event)
    return str(event)

def row_content_cb(event):
    print("Event", event)
    return str(event)

b = pn.widgets.Button(name="expanded")

t = pn.widgets.Tabulator(
    df,
    width=300,
    # row_content=row_content_cb,
    row_content=async_row_content_cb,
    # embed_content=True,
    # expanded=[1],
)

b.on_click(lambda e: setattr(t, 'expanded', [0]))

pn.Row(b, t).servable()

Draft as I'd like to test it in an app with a more complex row_content function.

[philippjfr]

Suggested change

	placeholder.object = resolved
	placeholder.loading = False
	placeholder.param.update(object=resolved, loading=False)

[philippjfr]

Think you have to make these tests async as well

Suggested change

	def test_tabulator_expanded_content_async(document, comm):
	async def test_tabulator_expanded_content_async(document, comm):

[philippjfr]

Suggested change

	def test_tabulator_content_embed_async(document, comm):
	async def test_tabulator_content_embed_async(document, comm):