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

Allow reusing Tag instances.

Remove GLib dependency from some headers.

Invoke decoder_initialized() in the libFLAC metadata callback.  This
merges code from the FLAC and the OggFLAC decoder plugin into the
common library.

This feature has been moved to the "flac" playlist plugin.

This function replaces the replay_gain_info parameter for
decoder_data().  This allows the decoder to announce replay gain
changes, instead of having to pass the same object over and over.

This patch prepares support for floating point samples (and probably
other formats).  It changes the meaning of the "bits" attribute from a
bit count to a symbolic value.

Remove the audio_format attribute, add "frame_size" instead.  The
audio_format initialization and check is moved both to
flac_data_get_audio_format().

Don't update a float timestamp, this will make imprecisions add up
after a while.  We already have the number of the current frame, let's
just calculate the float timestamp from that for every decoder_data()
command.  For this, we need to add the attribute "first_frame", for
CUE sheet songs.

Removed the "bit_rate" attribute from the flac_data struct.  Pass the
number of bytes since the last call to flac_common_write(), and let
it calculate the bit rate.

We don't want to work with floating point values if possible.  Get the
integer number of frames from the FLAC__StreamMetadata_StreamInfo
object, and convert it into a float duration on demand.  This patch
adds a check if the STREAMINFO packet has been received yet.

We need this for more exact end-of-subsong detection for CUE files.

Simple (up-rounding) integer division is good enough.  We're casting
the result back to an integer anyway.