Convenient method that behave differently than Read, and that will be used by
tag scanners.

This method will return in case of error, if the whole data is read or is EOF
is reached.

Reduce header dependencies.

Remove the "whence" parameter that is not actually necessary, and only
complicates the InputStream implementations.

Replace InputPlugin attributes.

Replaces the method Close().

Merge some duplicate code.

Decouple some more from GLib.

Allows forward-declaration.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.