fix(gridMenu): adding user customItems in GridMenu longer showing, fix #244

[ghiscoding]

closes issue #244

[codecov-io]

Codecov Report

Merging #245 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
+ Coverage   75.33%   75.38%   +0.06%     
==========================================
  Files         145      145              
  Lines        6966     6981      +15     
  Branches     2296     2303       +7     
==========================================
+ Hits         5247     5262      +15     
  Misses       1719     1719

Impacted Files	Coverage Δ
.../angular-slickgrid/extensions/gridMenuExtension.ts	100% <100%> (ø)	⬆️

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