This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Work around -Werror=catch-value.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

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

Fix breakage from commit bea5681f

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

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

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

Apparently, this makes old clang versions happy ("'alignas' attribute
cannot be applied to types).