Add UpdateQueueItem::id to keep track of the id in every item.
Replaces thhe hack in update_queue_push().

Fixes a few kludges and avoids GLib allocation.

Decouple some more from GLib.

Allows forward-declaration.

If we return false, their values are undefined.

Restore the "mpcdec" decoder plugin that was accidently broken since
commit 8ead8f7e.

Make the third argument to AC_CHECK_LIB() non-empty to disable its
default handler.

Replacing GLib macros.

Micro-optimization.