Info and plaintext C converter @image formatting

* tta/C/convert/converter.c (converter_txt_image_text): add.

* tta/C/convert/convert_to_info.c (protect_image_string)
(info_format_image, info_format_image_element),
tta/C/convert/convert_to_plaintext.c (PLAINTEXT_FORMAT_FUNCTIONS)
(plaintext_image_formatted_text, plaintext_format_image_element)
(plaintext_functions, convert_to_plaintext_internal),
tta/C/main/converter_types.h (STRING_LINE_COUNT): implement Info and
plaintext image formatting, add dispatch.  In Info, the result is
incorrect because NULL is used as a quoting character and the code is
not adapted to that.

* tta/C/convert/convert_to_info.c (info_format_node): check that
node_directions is set before accessing.