Implement single and multi-selection in Vega pane

[philippjfr]

Implements #3398

[codecov]

Codecov Report

Merging #3470 (ba4e172) into master (3d196cb) will decrease coverage by 0.03%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master    #3470      +/-   ##
==========================================
- Coverage   83.17%   83.13%   -0.04%     
==========================================
  Files         199      199              
  Lines       27092    27093       +1     
==========================================
- Hits        22533    22525       -8     
- Misses       4559     4568       +9     

Impacted Files	Coverage Δ
panel/pane/vega.py	79.77% <44.44%> (-3.27%)	⬇️
panel/io/reload.py	69.56% <0.00%> (-2.18%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/io/server.py	78.04% <0.00%> (-0.22%)	⬇️
panel/io/state.py	70.38% <0.00%> (-0.22%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️
panel/io/callbacks.py	72.99% <0.00%> (+0.52%)	⬆️

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