Be more allowing when looking for menu-bar items

Be more allowing when looking for menu-bar items

* src/keymap.c (lookup_key_1): Factor out function from
Flookup_key.
(Flookup_key): Be case insensitive, and treat spaces as dashes,
when looking for Qmenu_bar items.  (Bug#50752)

* test/src/keymap-tests.el
(keymap-lookup-key/mixed-case)
(keymap-lookup-key/mixed-case-multibyte)
(keymap-lookup-keymap/with-spaces)
(keymap-lookup-keymap/with-spaces-multibyte)
(keymap-lookup-keymap/with-spaces-multibyte-lang-env): New tests.