Added LayoutUpdated event.

[grokys]

Fixes #825.

[codecov-io]

Codecov Report

Merging #840 into master will increase coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff            @@
##           master    #840      +/-   ##
=========================================
+ Coverage   61.39%   61.4%   +<.01%     
=========================================
  Files         530     530              
  Lines       20126   20127       +1     
  Branches     2865    2866       +1     
=========================================
+ Hits        12357   12358       +1     
  Misses       6947    6947              
  Partials      822     822

Impacted Files	Coverage Δ
src/Avalonia.Layout/Layoutable.cs	91.82% <ø> (-0.45%)	❌
src/Avalonia.Base/Threading/JobRunner.cs	45% <ø> (+2.5%)	✅

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