This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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

Caused by commit 0b4fa41a

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Decouple some more from GLib.

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

Replaces GLib's GError.

Eliminate input_stream_init() and input_stream_deinit().

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