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

[ghiscoding]

• currently only works by click event on sub-menus, I tried to implement it with mouseover/mouseout but it's a lot more complex (we need to know if we are mousing over the correct sub-menu or over something else and this and that... wow just too much work)... so click should be enough for now, even though it's slightly less user friendly

TODOS

• requires PR feat: add sub-menu(s) to CellMenu & ContextMenu plugins #867 to be merged for this PR to not fail
  • build fails because subMenuTitle and subMenuTitleCssClass were added in previous PR
• add sub-menus to Grid Menu control
• add Cypress E2E tests
• add subMenuTitle & subMenuTitleCssClass to optionally add sub-menu title & custom styling
• opening a sub-menu then open a sub-menu from another tree doesn't close previous sub-menu that becomes orphan