fix(styling): cell/context menus get re-position below the grid

[ghiscoding]

• not an issue with Bootstrap but other frameworks like Bulma had this issue which it was overriding the SlickGrid cell-menu/context-menu styling because of these menus use generic css dropdown/dropup classes

[codecov]

Codecov Report

Merging #479 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            master      #479    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          148       148            
  Lines         9542      9542            
  Branches      3321      3185   -136     
==========================================
  Hits          9542      9542            

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