External state pages

[eagoetz]

This PR adds an ExternalStatePage in order to have a tab with states that comes from an external generated process.

[codecov]

Codecov Report

Attention: Patch coverage is 28.98551% with 49 lines in your changes missing coverage. Please review.

Project coverage is 49.11%. Comparing base (720bd0c) to head (b537d06).
Report is 17 commits behind head on master.

Files with missing lines	Patch %	Lines
gwsumm/tabs/builtin.py	28.99%	49 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #426      +/-   ##
==========================================
- Coverage   49.64%   49.11%   -0.53%     
==========================================
  Files          60       60              
  Lines        8823     8858      +35     
==========================================
- Hits         4380     4350      -30     
- Misses       4443     4508      +65     

Flag	Coverage Δ
Linux	49.11% <28.99%> (-0.53%)	⬇️
macOS	49.11% <28.99%> (?)
python3.10	49.11% <28.99%> (-0.53%)	⬇️
python3.11	49.11% <28.99%> (-0.53%)	⬇️
python3.9	49.11% <28.99%> (-0.53%)	⬇️

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