mpc.el: Misc tweaks.

mpc.el: Misc tweaks.

* lisp/mpc.el: Require `cl-lib` at run-time.
Don't require `subr-x` any more.
(mpc-status-callbacks): Improve docstring.
(mpc-cover-image-find): Avoid `and-let*`.
(mpc-cover-image-p): Use `member-ignore-case` to recover the
behavior before last patch.
(mpc-cover-image-find, mpc-cover-image-p): Move to later in the
file to avoid compiler warning.
(mpc-format, mpc-notifications-title, mpc-notifications-body):
Tweak docstring.
(mpc--notifications-format): Create only one temp buffer and use
the faster `cl-some` since we know the arg is a list.
(mpc-notifications-notify): Prefer `when-let*` since all the other
`*-let` we use are also of the `let*`family.

(cherry picked from commit 4690f2f44d38d64d8ebfea98c085844a5723df0f)