multiple menu-items (not just the 'create' menu-group).
Moved all the main menu/context menu generation implementation in the
'menu' javascript menu.
In this implementation, if more than one menu-items specify same type
of categories, they will be created withing that group, otherwise - it
will be created separately (unless 'single' property of that category is
set to true).
We can also provide icon, priority, separator(s) above/below it for the
individual sub-menu too using pgAdmin.Browser.add_menu_category
function(...).
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