Extend the @Prop annotation with observable parameter

[realityforge]

This makes it possible to control which props are observable. It also fixes a bug that failed to propagate props changes

[codecov-io]

Codecov Report

Merging #7 into master will decrease coverage by 0.17%.
The diff coverage is 87.27%.

@@             Coverage Diff              @@
##             master       #7      +/-   ##
============================================
- Coverage     93.14%   92.97%   -0.18%     
- Complexity      718      741      +23     
============================================
  Files            16       16              
  Lines          2042     2092      +50     
  Branches        348      360      +12     
============================================
+ Hits           1902     1945      +43     
- Misses           45       47       +2     
- Partials         95      100       +5

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/react4j/processor/PropDescriptor.java	91.66% <100%> (+0.36%)	25 <1> (+1)	⬆️
...rc/main/java/react4j/processor/ReactProcessor.java	89.19% <83.33%> (-0.17%)	289 <12> (+14)
...sor/src/main/java/react4j/processor/Generator.java	96.52% <88.57%> (-0.33%)	209 <4> (+8)

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