feat: add sub-menu(s) to HeaderMenu plugin

[ghiscoding]

• currently only works by click event on sub-menus, the mouseover will be implemented in a future PR

TODOs

• add full unit test coverage
• add Cypress E2E tests
• it might be nice to add auto-align top/bottom especially for sub-menus

[codecov]

Codecov Report

Merging #1158 (637364c) into master (8999297) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1158   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          245       245           
  Lines        17075     17145   +70     
  Branches      6151      6182   +31     
=========================================
+ Hits         17075     17145   +70     

Files	Coverage Δ
packages/common/src/extensions/menuBaseClass.ts	100.00% <100.00%> (ø)
packages/common/src/extensions/slickGridMenu.ts	100.00% <ø> (ø)
packages/common/src/extensions/slickHeaderMenu.ts	100.00% <100.00%> (ø)