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

Validate early, so we avoid the rollback if an error occurs.

Skip the LookupRemoteTag() call if the MoveId() call fails.

Eliminates lots of implicit std::string temporaries.

This reverts commit 6ed4aff4.  The
Meson wrap is broken on non-x86, because it unconditionally includes
<cpuid.h> which is an x86 only header.

This way, URI schemes supported by arbitrary storage plugins are
allowed.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1270

Fixes crash bug.

This should translate to ACK_ERROR_ARG instead of ACK_ERROR_UNKNOWN.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1265

This way, we know whether we're explicitly "paused" or "not yet
activated because the ring buffer hasn't been filled yet".

As per Meson 0.56.0:
 https://mesonbuild.com/Machine-files.html#properties

Fixes the build with libexpat from Meson wrap.

Not necessary anymore since Meson 0.50.0:
 https://mesonbuild.com/Release-notes-for-0-50-0.html#libdir-defaults-to-lib-when-cross-compiling

Requires Meson 0.56.0:
 https://mesonbuild.com/Release-notes-for-0-56-0.html#per-subproject-warning_level-option

Requires Meson 0.54.0:
 https://mesonbuild.com/Release-notes-for-0-54-0.html#per-subproject-default_library-and-werror-options

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

Necessary since commit 6acf81d5

Let cmake know that we're cross-crompiling.

Prepare to add some code around the _build() call.

Eliminate conflict with other Project classes.