See commit 71f0ed8b

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

An explicit destructor prevents usage of implicit move operators, even
if it's empty.  Therefore, declaring a defaulted destructor with GCC
attribute "noinline" does what we want without preventing those
implicit operators.

This reverts commit 67b7d464.

Turns out I was wrong, and mentioning these does make a difference:
the implicit move constructor is not defined in the presence of a
user-declared destructor.

Sometimes, it's useful to construct a DetachedSong from a LightSong
even without having real_uri initialized.

Mentioning these doesn't make a difference.

This method is used by DecoderControl::IsCurrentSong(), which is used
by the player thread to check whether the current decoder instance can
be reused to seek.  When switching to another song in the same CUE
sheet, previously DetachedSong::IsSame() returned true, and thus the
old decoder instance was used for the new song, not considering the
new end_time.  This led to the old decoder quickly quitting.

Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Use Storage::MapUTF8() internally, don't use global variables.

Reduce bloat.

Prepare for UPnP songs that retain there database identity.

Suppresses gcc warnings.

Remove unnecessary explicit code.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

Don't create an intermediate Song instance when all we want is a
DetachedSong.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.