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

This was a no-op previously, but this operation makes no sense.

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

Use MoveRange() instead of MoveId().

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.

The "move" command doesn't allow open-ended ranges because they don't
make a lot of sense; moving an open-ended range is only possible if
the destination index is before the range, and in that case, the
client should be well aware how many songs there are.

Closes https://github.com/MusicPlayerDaemon/MPD/pull/1057

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

Prepare to convert "partition" to a mutable pointer.

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).

Previously the id was being returned only when a position wasn't
specified.