Improve support for exporting View and Target to_spec

[philippjfr]

This rounds out our ability to declare Lumen components programmatically. We can now build View and Target components programmatically and then export them to a specification.

[codecov-commenter]

Codecov Report

Merging #365 (e9cdd0b) into master (18878b1) will increase coverage by 0.69%.
The diff coverage is 80.23%.

@@            Coverage Diff             @@
##           master     #365      +/-   ##
==========================================
+ Coverage   66.14%   66.84%   +0.69%     
==========================================
  Files          72       72              
  Lines        7710     7863     +153     
==========================================
+ Hits         5100     5256     +156     
+ Misses       2610     2607       -3     

Impacted Files	Coverage Δ
lumen/base.py	79.72% <0.00%> (+1.42%)	⬆️
lumen/ui/sources.py	48.46% <0.00%> (-0.57%)	⬇️
lumen/state.py	62.14% <6.66%> (-3.37%)	⬇️
lumen/views/base.py	62.62% <59.09%> (+1.03%)	⬆️
lumen/pipeline.py	57.95% <80.00%> (+4.90%)	⬆️
lumen/target.py	78.22% <83.33%> (+1.21%)	⬆️
lumen/tests/test_target.py	100.00% <100.00%> (ø)
lumen/tests/views/test_base.py	100.00% <100.00%> (ø)
... and 2 more

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[github-actions]

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