Closes #155

Move its code to DeferEvent instead.

Allows defining a list of supported audio formats, and allows
switching on and off DoP with certain formats.

This is a first rough draft.  The setting syntax and its semantics may
still be redesigned.

Needed for WStringView.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

Our local EventLoop instance was never run; it was not usable.

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

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

We will reuse these tags elsewhere.

The first step to multi-player support.  Not much, just a dummy
command.

Eliminate global variables.