use full metadata tag name for variable substitution

[zisoft]

It turned out that using only the last part of the metadata tag name is not sufficient and in some cases it can be ambiguous.

In #18976 it breaks watermark settings for old edits.

Solution is to use the full metadata tag name for the variable name:

This requires a documentation change as well, I will provide a PR on dtdocs for that.

fixes #18976

[TurboGit]

In the change I see that you're using the full tagname. Fine. But at the same time you've also removed the upper-case change, can you comment on that?

TIA.

[zisoft]

Yes, when importing images Xmp.dc.creator is read from the metadata.
Using uppercase variable names for the metadata tags won't match anymore.

[TurboGit]

I see now, it works for me. Thanks.