devel: Document emacs keybindings

This adds a file under devel listing all the keybindings sorted by key
in the main three modes (search, show and tree).

To reduce clutter it only lists the single key "unmodified"
keybindings: I think all our uses of prefixes and modifiers are
natural variants of the unmodified binding (eg M-n compared to n in
show mode)

This should make easier to see what keybindings are available when
adding new features.

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