Make Param._mapping public

[philippjfr]

Only makes the attribute public for now, the suggestions in #1348 are still valid and that issue will stay open.

Supersedes #1322
Fixes #620

[codecov]

Codecov Report

Merging #3173 (8267435) into master (f1502c6) will decrease coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #3173      +/-   ##
==========================================
- Coverage   82.98%   82.97%   -0.02%     
==========================================
  Files         192      192              
  Lines       25502    25506       +4     
==========================================
  Hits        21163    21163              
- Misses       4339     4343       +4     

Impacted Files	Coverage Δ
panel/param.py	86.83% <85.71%> (-0.21%)	⬇️
panel/io/reload.py	69.23% <0.00%> (-2.20%)	⬇️
panel/tests/command/test_serve.py	83.54% <0.00%> (-1.46%)	⬇️
panel/pane/idom.py	27.40% <0.00%> (+0.20%)	⬆️

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