Link widgets with same name during embed

[philippjfr]

Fixes #806
Fixes #1083

[codecov]

Codecov Report

Merging #1543 into master will decrease coverage by 0.03%.
The diff coverage is 75.43%.

@@            Coverage Diff             @@
##           master    #1543      +/-   ##
==========================================
- Coverage   85.63%   85.60%   -0.04%     
==========================================
  Files         146      146              
  Lines       16081    16161      +80     
==========================================
+ Hits        13771    13834      +63     
- Misses       2310     2327      +17     

Impacted Files	Coverage Δ
panel/io/callbacks.py	26.02% <15.62%> (-3.39%)	⬇️
panel/reactive.py	86.43% <94.11%> (+1.06%)	⬆️
panel/io/embed.py	86.87% <100.00%> (+0.88%)	⬆️
panel/tests/io/test_embed.py	99.39% <100.00%> (+0.04%)	⬆️
panel/tests/test_links.py	98.88% <100.00%> (+0.06%)	⬆️

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