Closes https://github.com/MusicPlayerDaemon/MPD/issues/1112

Regression by commit a22d1c88

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1097

Exmaple -> Example

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

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.

It is useless, because we're always checking pa_stream_is_corked().

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1022

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1031

Commit 79b23663 added the field `skip`
to support unaligned reads, but set the `offset` field to a wrong
value.  This resulted in miscalculation of `remaining`, causing
an assertion failure.

The fix is to assign `offset` the correct value, but consider the
`skip` value in the assertion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1067

After a Seek() to an odd offset, some data needs to be skipped from
the start of the block, and reading right into the given buffer
doesn't work.

Conductor was incorrectly saved to Performer tag in MPD database

Fixes Windows compatibility.

The Google overlords require me to change to 29 or else I can't upload
new releases to Google Play.

 https://developer.android.com/distribute/best-practices/develop/target-sdk