generator: Add option to validate GAPI XML against an XSD schema

With a new --schema option, you can specify the path to an XSD file, and
all GAPI XML files will be validated against this schema, including the
files given through the --include option.
Remove obsolete fix-ups for needs_ref attribute

The needs_ref attribute hasn't been used by the generator for quite some
time now.
pango: Remove invalid fix-up for pango_layout_get_lines_readonly

This fix-up applies the element_type attribute to the method itself,
which is invalid. The correct fix-up that applies it to the return-type
is already there.
Add and use gapi.xsd to validate GAPI XML files

The structure of a GAPI XML file is now defined by the XML schema in
gapi.xsd.

This XSD is now used by the generator to do a first sanity check on an
XML file before trying to generate code from it. Each generatable object
still does its own validation.

The XSD can also be seen as a documentation of the GAPI XML format, and
can be used by third-parties that produce GAPI XML to validate their
output.