Must be initialized for the check in SetVolume().

The writing and reading of metadata involves lots of different programs
and libraries. Therefore it is prudent to point out how exactly MPD
receives metadata. Ideally this helps to point users to the right place
if their tags are not picked up correctly.

Don't return early from ScanOpusTags() if only
TagHandler::WantPicture() is set.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1413

lib/src/libmpdclient-2.19/meson.build:1:0: ERROR: Unknown options: "needs_exe_wrapper"

The "needs_exe_wrapper" option was incorrectly set under
[built-in options] rather than [properties].

Improves the changes made in 57687779 by
using AudioManager.ACTION_AUDIO_BECOMING_NOISY rather than listening for
wired headset unplug events or Bluetooth headset disconnect events. This
method is more flexible, allowing the feature to work on other types of
audio output device, as well as Bluetooth devices that don't set their
device class correctly. This change also has the benefit of being more
responsive, pausing the audio before it is rerouted to the built-in
speaker.

https://developer.android.com/guide/topics/media-apps/volume-and-earphones

Signed-off-by: Rosen Penev <rosenp@gmail.com>

Fixes assertion failure when the submit queue is empty.

Fixes MixRamp failures when a MP3 file has two ID3 tags, one of them
without MixRamp.