Auto-toggle CheckableItem

[jtreminio]

Per this comment it makes sense for a CheckboxItem to auto-toggle itself (either on or off) vs forcing developer to implement this themselves.

CheckableItem::getSelectAction() returns a callable within a callable. The outer callable does the auto-toggling, the inner callable is the user-defined code.

The CheckableItemTest::testGetSelectAction() reflects this stacking.

[codecov-io]

Codecov Report

Merging #193 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #193      +/-   ##
============================================
+ Coverage     93.05%   93.06%   +0.01%     
  Complexity      491      491              
============================================
  Files            27       27              
  Lines          1483     1486       +3     
============================================
+ Hits           1380     1383       +3     
  Misses          103      103

Impacted Files	Coverage Δ	Complexity Δ
src/MenuItem/CheckableItem.php	87.5% <100%> (+0.83%)	19 <0> (ø)	⬇️

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[AydinHassan]

Nice, was going to suggest this myself!