Related issue #1882552: Get rid of menu_list_system_menus()
They are needed to select parent menu link all over core.
Suppose we need deeper refactoring of menu link selection widget - for example:
[select menu] - ajax loads dependent list of it's menu-items
[select menu item]
Looks like there's only few forms to add menu links, so we should just unify this widget to MenuLinkFormController
menu_edit_item() - hopefully gone with #916388: Convert menu links into entities to form controller
Updated issue summary.
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