input: add new keys: Back, Tools, ZoomIn, ZoomOut

These were the only keys missing to support mapping all keycodes on a
[popular RF Remote][1] used with Linux HTPC apps.

Note that X11/XWayland + xkbcomp still warns about keycodes > 255, due
to the 8-bit limit in Xorg but `mpv` on Wayland is able to handle these.
For X11 users, there are [a couple options][2]:

- [Gianni Ceccarelli's patched `xf86-input-evdev`][3]
- [Use udev hwdb to map scancodes to keycodes][4]

[1]: https://www.mythtv.org/wiki/Air_mouse_rf_remote
[2]: https://unix.stackexchange.com/a/436233/7688
[3]: https://www.thenautilus.net/SW/xf86-input-evdev/
[4]: https://wiki.archlinux.org/title/Map_scancodes_to_keycodes