Fix hotkeys in prefs when unset

If hotkeys are not set, the label is (None). When gtk_accelerator parses
it, it fails and set keysym to zero. In this case, we shouldn't feed
that to XKeysymToKeycode(), since it will indeed return a value (204)
which will be saved in the prefs.
Add a (geek) way to reset hotkeys

Maybe using <Ctrl>C is not very user friendly, but it's easy and
intuitive to me.

TODO: translation for the new line in the UI
Grey out hotkey volume step when disabled

Also rename hotkey_spin in hotkey_vol_spin for consistency with
hotkey_vol_label.