bug#21184: 25.0.5; Use a monospace font the keybind descriptions on the menu-bar

[Artur Malabarba]

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.16.4)

In the menu-bar menus, next to most entries is listed a keybind that
can be used to invoke that command. This keybind, like the rest of the
menu, is shown in a variable-width font.

IMO it would be best (if possible) to use a monospace font on the
keybinds. This should look better because they would be vertically
aligned, and because (unlike the rest of the menu) the key descriptors
represent data, not prose.

Not sure if it's possible, though.