adg: bare export API adg_canvas_export()

Added also the helper function adg_type_from_filename() that tries to
guess the surface type from the file name. Modified adg-demo to use the
new APIs accordingly.

WARNING: one argument of adg_canvas_export() is a cairo_surface_type_t.
Actually cairo does not export such type via gobject-introspection so
that API will not be available to language bindings. I made a pull
request to add support to that (and more) types:
https://github.com/GNOME/gobject-introspection/pull/1
The API will be available only when that PR will be merged upstream.

Closes issue #106: http://dev.entidi.com/p/adg/issues/106/
i18n: update italian translations

Also, corrected some minor mistakes in cpml-demo localization.
doc: annotate AgdGtkArea:canvas-changed parameters

For some unknown reason, gobject-introspection is unable to get the
proper C type for the old_canvas argument of the "canvas-changed" signal
of AdgGtkArea object. Explicitely specifying its C type solved the
problem.

Closes issue #102: http://dev.entidi.com/p/adg/issues/102/