[NUI][AT-SPI] text: add "GetRangeExtents" interface

[NUI][AT-SPI] text: add "GetRangeExtents" interface

This interface will be used for getting MBR(Minimum Bounding Rectangle)
with following usage on the AT client side.

  cc = atspi_text_get_character_count(text, NULL);
  rect = atspi_text_get_range_extents(text, 0, cc, ATSPI_COORD_TYPE_WINDOW, NULL);