Found with readability-const-return-type
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Eliminate a use of GetGlobalConfig().

All it does is "sometimes" close the mixer, but that is also done in
the destructor.

This is implicitly done by the destructor.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Yet another C-style vtable replaced with C++.

Hide struct FilteredAudioOutput from the plugins, preparing for hiding
MPD's core internals.

Avoid accessing FilteredAudioFormat attributes directly.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

Moved code from AudioOutputControl::InternalOpen2().

struct AudioOutput should not know or care about multi-threading.

Prepare for moving the AudioOutputSource to class AudioOutputControl.

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