Coverity discovered that the Pulse plugin could throw exceptions from
Pause(), but that method was marked "noexcept" because its caller was
not designed to catch exceptions.  So instead of avoiding exceptions
(by catching and logging them in each and every implementation), let's
allow them, and do the catch/log game in the MPD core.

Fixes regression from commit 31bad5f7:
if Pause() returns true, the output thread is running a busy loop,
causing 100% CPU usage, instead of just closing the output.

Closes #105

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.

Avoid accessing FilteredAudioFormat attributes directly.

Configure the AudioFormat after opening the encoder, because only now
we know the final format.

This order will be necessary for proper AudioFormat initialization,
because the encoder may change the format.

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

This call cannot fail, but needs cleanup if others fail.  By moving it
to the end, we eliminate some cleanup.

Moved code from AudioOutputControl::InternalOpen2().

Circumvent filter_new() with a fake ConfigBlock.