Closes https://github.com/MusicPlayerDaemon/MPD/issues/1337

This enhances readability in sidebar, especially for "User’s Manual" and
"Protocol" pages

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1328

Add an option to the UPnP database plugin to configure which interface
is used by upnp to discover servers.

upnp by default selects the first interface that is not loopback, which
in some cases might not be the desired interface. For example if wanting
to access a DLNA server over a VPN connection.

The "interface" option can now be set to the name of the desired
interface to achieve this.

The default behaviour remains unchanged.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1106

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1285

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1297

This commit also increases the PROTOCOL_VERSION so clients can detect
the availability of the feature.

Whoops, I misplaced this one.

Another one from https://github.com/MusicPlayerDaemon/MPD/issues/888

Closes https://github.com/MusicPlayerDaemon/MPD/issues/888

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1115

Some users want certain clients to fully control playback, but do not
want them to be able to trigger database update.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1124

This reduces duplicates in the music database by hiding the original
song file when it is referenced by a CUE sheet.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1275

The implementation was removed 12 years ago in commit f6455d5f and
nobody missed it.

The existing implementation has been utterly broken forever; I cannot
explain what it actually does, but it doesn't do what the
documentation says.

Now, "+0" means "right after the current song" and "-0" means "right
before the current song".  Mnemonic: there are zero songs between the
current song and the newly added song.

A similar feature was present long ago in MPD, but was deprecated in
version 0.16 because the implementation was broken.  This commit
re-adds the feature in a way that's well-defined and not broken.

Close https://github.com/MusicPlayerDaemon/MPD/issues/1221

I want to use per-subproject warning_level (0.56.0), default_library
(0.54.0).

NB: Check the sysconfdir setting to determine where mpd will look for mpd.conf; if you expect mpd to look for /etc/mpd.conf the sysconfdir must be '/etc' (i.e., not 'etc' which will result in mpd looking for /usr/local/etc/mpd.conf):
 
.. code-block:: none
 
 meson configure output/release |grep sysconfdir

If this is not /etc (or another path you wish to specify):

.. code-block:: none

 $ meson configure output/release -Dsysconfdir='/etc' ; meson configure output/release |grep syscon
  sysconfdir              /etc                                               Sysconf data directory