Prevents sending excessive value updates to the native side

[pulyaevskiy]

Fixes #11.

This addresses two issues:

1. Ensures that composing range is always set to the last known value for data sent to the native side.
2. Prevents sending remote updates if current value (including composing range) is the same the the last known value.

cc @stemuk

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #12      +/-   ##
=========================================
+ Coverage   91.79%   91.8%   +0.01%     
=========================================
  Files          32      32              
  Lines        2607    2612       +5     
=========================================
+ Hits         2393    2398       +5     
  Misses        214     214

Impacted Files	Coverage Δ
packages/zefyr/lib/src/widgets/input.dart	93.18% <100%> (+0.49%)	⬆️
packages/zefyr/lib/src/widgets/editable_text.dart	93.79% <0%> (+0.09%)	⬆️

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