postscript-dev authored Aug 15, 2021 and Jens Georg committed Aug 15, 2021

The library is moving to a new API, using GError to report errors and an
`_try` prefix in the function name. Some existing functions already used
GError but were not using the `_try` prefix.

+ Add missing `_try` versions of the functions and deprecate non-`_try` ones
+ Add new functions to gtk-docs and move non-`_try` ones to subsection
+ Add new functions to test_presence_of_expected_methods() in Python testing
+ Update gexiv2-tool with `try_open_path()` to prevent deprecation warning
+ NULL to nullptr as part of modernisation
+ Update gexiv2-tool with `try_get_xmp_namespace_for_tag()` to prevent
  deprecation warning
+ Python testing
+ Replace `g_ascii_strncasecmp()` with `strncmp()` as this is case sensitive
+ Add error message in functions, when filtering by @tag and does not match
  any of "Exif.", "Iptc." or "Xmp."
+ Update `Since` and `Deprecated` fields in function docs

Closes !54