pangomm: New API in pangomm 1.6

# New API in pangomm 1.6

Member Pango::Context::get_font_map () const

Member Pango::Context::set_matrix (const Matrix& matrix)

Member Pango::FontMetrics::get_strikethrough_position () const

Member Pango::FontMetrics::get_strikethrough_thickness () const

Member Pango::FontMetrics::get_underline_position () const

Member Pango::FontMetrics::get_underline_thickness () const

Member Pango::Layout::get_ellipsize () const

Member Pango::Layout::set_ellipsize (EllipsizeMode ellipsize)