Wake up the OutputThread only if it hasn't already been woken up and
if it isn't already in the playback loop.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Use only ConfigData.hxx in plugin sources to reduce header
dependencies.

Replaces GLib's GError.

Look up top-level config options by enum (= integer), not by name
string.

Implements Mantis ticket 0003340.

No, really!  This fixes a regression of commit 74617389, which
changed the order of filter plugins.

Don't require the caller to provide them.

If output plugin fails to init it will try to call ao_base_finish() immediately,
which segfaults because replay gain filters are not initialized yet and contain
garbage values.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

The output thread could hang indefinitely after finishing CANCEL,
because it could have missed the signal while the output was not
unlocked in ao_command_finished().

This patch removes the wait() call after CANCEL, and adds the flag
"allow_play" instead.  While this flag is set, playback is skipped.
With this flag, there will not be any excess wait() call after the
pipe has been cleared.

This patch fixes a bug that causes mpd to discontinue playback after
seeking, due to the race condition described above.