permit nested live

[willmcgugan]

Allow nesting of Live instances.

Currently if you attempted to activate a Live when one was already running, then it would raise an error.

This update keeps a stack of Live objects on the console, and adds nested renderables to the top-most Live. The effect is that the nested Live appears below the first one(s). I suspect this is what most people will want.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.

Project coverage is 97.89%. Comparing base (45cc90b) to head (5a2db8f).
Report is 83 commits behind head on master.

Files with missing lines	Patch %	Lines
rich/live.py	55.55%	8 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3768      +/-   ##
==========================================
- Coverage   97.98%   97.89%   -0.10%     
==========================================
  Files          74       74              
  Lines        8142     8155      +13     
==========================================
+ Hits         7978     7983       +5     
- Misses        164      172       +8     

Flag	Coverage Δ
unittests	97.89% <65.21%> (-0.10%)	⬇️

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