Wrap pango_shape_with_flags()

* pango/src/glyphstring.[ccg|hg]: Add constructor
GlyphString(const Glib::ustring& item_text,
const Glib::ustring& paragraph_text, const Analysis& analysis,
ShapeFlags flags = SHAPE_NONE).
* pango/src/item.[ccg|hg]: Add enum ShapeFlags and
shape(const Glib::ustring& item_text, const Glib::ustring& paragraph_text,
ShapeFlags flags = SHAPE_NONE).
* tools/m4/convert_pango.m4: Add conversions for ShapeFlags.

See #9