Complete `_try` version of the interface

The library is moving to a new API, using GError to report errors and an _try prefix in the function name. This MR converts the remaining functions to the new format and adds some error checking.

• 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
• Add test_presence_of_expected_methods() for GExiv2PreviewProperties and GExiv2PreviewImage in Python testing
• Add/Extend missing try/catch blocks. Report errors to GError** param
• Add error message when filtering by @tag ("Exif."/"Iptc."/"Xmp.") and supplied tag does not match.
• Add error checking/testing to _metadata_try_get_xmp_namespace_for_tag()
• NULL to nullptr as part of modernisation