Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Make it "const".

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.