Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Those are loaded with the "curl" input plugin, and this one is
"expensive", because it needs to send a new HTTP request with Range
header for each seek.

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

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.

Seeking on NFS or SMB is cheap.  Actually, only HTTP streams are
expensive to seek.  This enables a few features on NFS/SMB files, for
example Ogg tags.

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

Allow compiling test programs with only selected plugins.

Replace InputPlugin attributes.

Replaces the method Close().

Merge some duplicate code.

Decouple some more from GLib.

Replaces GLib's GError.

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