New convenience functions are provided which simplify
manipulation of key bindings and menu item visibility
configuration strings. AbstractMenu#pack_key_binding
and AbstractMenu#unpack_key_binding turn a path/key
map into a single string and back. The string format
is the same than for the key-binding configuration key.
The same is provided for the menu item visibilily
with AbstractMenu#pack_menu_item_visible and
Abstract#unpack_menu_item_visible.
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