InputStream::Read() and InputStream::Seek() are called with the mutex
locked.  That means the implementation must not block, or unlock the
mutex before calling into blocking code.

Previously, a slow CD drive could stall the whole MPD process,
including the main thread, due to this problem.

Closes #149

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

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

This special case was useless.  Fixes the dreaded "Unrecognized URI"
error message when a file does not exist.

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

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.