Add support for rendering panels inside Tabulator rows

[philippjfr]

Adds two parameters to Tabulator to support adding additional content inside a Tabulator row, which acts like an accordion.

• expanded: The currrently expanded rows
• row_content: A function which is given a DataFrame row as input and should output a Panel object
• embed_content: Whether to embed all the contents or to dynamically fetch them when the row is expanded (default)

Implements #2761

def row_detail(row):
    return row.to_frame()

pn.widgets.Tabulator(
    value=pd.DataFrame([(0, 1), (1, 2)]),
    row_content=row_detail, sizing_mode='stretch_height'
)

world = geopandas.read_file(geopandas.datasets.get_path('naturalearth_lowres'))
pn.widgets.Tabulator(world, row_content=lambda row: geopandas.GeoDataFrame(row.to_frame().T).hvplot())

• Add tests
• Update Tabulator reference gallery
• Add gallery example
• Ensure children are cleaned up

[MarcSkovMadsen]

This is exactly the functionality I've been using via Angular. Looks great.

[codecov]

Codecov Report

Merging #2762 (ed9a9cc) into master (1bd5987) will increase coverage by 0.06%.
The diff coverage is 96.22%.

@@            Coverage Diff             @@
##           master    #2762      +/-   ##
==========================================
+ Coverage   82.97%   83.03%   +0.06%     
==========================================
  Files         188      188              
  Lines       24162    24264     +102     
==========================================
+ Hits        20048    20148     +100     
- Misses       4114     4116       +2     

Impacted Files	Coverage Δ
panel/widgets/tables.py	86.55% <93.75%> (+1.05%)	⬆️
panel/models/tabulator.py	94.44% <100.00%> (+0.15%)	⬆️
panel/tests/widgets/test_tables.py	99.39% <100.00%> (+0.05%)	⬆️
panel/tests/pane/test_plotly.py	98.15% <0.00%> (-0.62%)	⬇️

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