Fix gexiv2_metadata_try_get_tag_multiple() xmpText/langAlt return values

Function returns incorrect values when tag is of type xmpText or
altLang (see Gexiv2 Issue GNOME/gexiv2#61).

Fix changes the return value format and is only implemented for
recently created gexiv2_metadata_try_get_tag_multiple(). For backwards
compatibility, the old format is preserved in
gexiv2_metadata_get_tag_multiple().

Changes:
* Create gexiv2_metadata_get_xmp_tag_multiple_deprecated() to preserve
the existing format.  Change gexiv2_metadata_get_tag_multiple() to use
this function.

* Fix gexiv2_metadata_try_get_xml_tag_multiple() to return correct
values when tag is of type xmpText and xmpAlt.

* Added documentation to gexiv2-metadata.h explaining returns values.

* Change NULL to nullptr as part of modernisation.
Status Job ID Name Coverage
  Build
passed #999708
build-job

00:03:13

 
  Test
passed #999709
test

00:01:37