[emms-help] BUG: keymap not set in emms-show-all-mode

[Fran Burstall]

I do M-x emms-show-all, inspect the data about the playing
track and hit 'q'.

What I expected: to quit the buffer.

What happens: nothing except a "buffer read-only error".

Reason: emms-show-all-mode-map is never set because it is
defvar'd after emms-mode is defined using
define-derived-mode which already populates
emms-show-all-mode-map from text-mode-map.

Solution: put the defvar form _before_ the
define-derived-mode form.

All this with latest MELPA emms and emacs 26.1.

Thanks for emms which I am enjoying very much (though taking
a while to understand).

---Fran