New experimental code, first draft - it works, but there's a lot left
to do.  Just look at all the TODO comments.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/975

Exmaple -> Example

release v0.22.6

If the constructor moves from an ExportedSong instance which refers to
somebody else's "Tag" instance, the newly constructed instance will
instead refer to its own empty "tag_buffer" field.  This broke
SimpleDatabase::GetSong(), i.e. all songs on the queue restored from
the state file or added using the "addid" command.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1089

release v0.22.5

Needed for `requestLegacyExternalStorage` (commit ca02fb77).

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

Catch errors like that early, before invalid ranges get passed to
internal MPD subsystems.

This is a workaround for the new scoped storage design in Android 11:

 https://developer.android.com/about/versions/11/privacy/storage

This needs a proper solution eventually, but this quick fix will do
until we change "targetSdkVersion" to 30.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1061

After commit 1afa33c3, an old bug was revealed:
SimpleDatabase::GetSong() constructs an ExportedSong instance by
moving the return value of Song::Export(), which causes the
LightSong::tag field to be dangling on the moved-from
ExportedSong::tag_buffer.  This broke tags from CUE sheets.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1070

Keep track of whether there is data being played, and don't call
AudioOutput::Drain() after Cancel() has been called already.

In this state, we can't make any progress.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1084

Drain only if it was requested explicitly.