fix(MenuItem): Merge Menu and MenuItem variables

[miroslavstastny]

Problem description

Menu and all its MenuItems share the same variables.
Menu passes its variables down to MenuItem as defaultProps. That means that if a consumer defines a variable directly on a MenuItem, it overwrites the variables passed from the Menu:

<Menu items={[
  {content: 'valid item'}, // ok, variables passed from Menu to the item
  {content: 'item', variables: {special: true}} // BUG, item variables replaces Menu variables, this menu item has broken styles
]} />

Solution

Instead of Menu passing its variables via defaultProps, it now does it through overrideProps and shallow-merges Menu variables with MenuItem variables.

• changelog

Fixes #1392.

[codecov]

Codecov Report

Merging #1447 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   73.27%   73.29%   +0.01%     
==========================================
  Files         802      802              
  Lines        6025     6028       +3     
  Branches     1775     1775              
==========================================
+ Hits         4415     4418       +3     
  Misses       1604     1604              
  Partials        6        6

Impacted Files	Coverage Δ
packages/react/src/components/Menu/Menu.tsx	71.05% <100%> (+2.48%)	⬆️

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[codecov]

Codecov Report

Merging #1447 into master will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   73.46%   73.67%   +0.21%     
==========================================
  Files         807      807              
  Lines        6086     6089       +3     
  Branches     1772     1753      -19     
==========================================
+ Hits         4471     4486      +15     
+ Misses       1610     1598      -12     
  Partials        5        5

Impacted Files	Coverage Δ
...react/src/components/Toolbar/ToolbarRadioGroup.tsx	100% <ø> (+30.76%)	⬆️
packages/react/src/components/Toolbar/Toolbar.tsx	100% <ø> (+47.05%)	⬆️
packages/react/src/components/Menu/Menu.tsx	71.05% <100%> (+2.48%)	⬆️

---

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