[WIP] Xmp toolkit v2023-12 submodule

[serghov]

This merge request attempts to pull XMP-Toolkit-SDK as a submodule. Due to the complicated and messy build system in XMP-Toolkit-SDK, I avoid using it and instead use the existing in-tree build system with some modifications.

I have made several hacky workarounds:

1. There is a hardcoded include for /third-party/expat/lib/expat.h in XMP-Toolkit-SDK. Since we pull expat as an external dependency, I had to copy expat.h to a fake third-party/expat/lib/ directory in the build folder at generation time. I am not quite sure if the same will be possible with meson.
2. MD5Update has a slightly different signature, so I used a const_cast. I expect to get rid of this when I do a refactoring.
3. SXMPMeta::DeleteNamespace seems to be deprecated in this version, which needs further investigation.

To-do:

• Add all compile definitions for different platforms that XMP-Toolkit-SDK requires, e.g., WIN_ENV, WIN_UNIVERSAL_ENV, etc.
• Integrate with meson.
• Refactor xmp.cpp and all associated files.
• Fix unit tests
• Fix github actions to clone with submodules

[serghov]

There are some changes in behavior of XMP-Toolkit-sdk in newer versions.

One major change I encountered is setting of aliased properties. Previously as far as I understand sdk did not care about aliasing whatsoever, the newer version do and attempt to handle them according to specifications.

Specific XMP implementations may treat a
property in one namespace as equivalent to a property in another namespace. However, to foster interchange,
applications must always write the standard, “base” form of the property.

XMPSpecificationPart2.pdf page 8

I found this comment in sources

  // According to the XMP specification, Xmp.tiff.ImageDescription is an
  // alias for Xmp.dc.description. Exiv2 treats an alias just like any
  // other property and leaves it to the application to implement specific
  // behaviour if desired.

Handling aliases in exiv2 was left to application developers, which is fine except new versions of XMP-Toolkit-sdk attempt to handle aliases under the hood, although I suspect they might have a bug or two in there. I will try to figure out a solution upstream.

So with this patch exiv2 will automagically set the value of the property alias is pointing to, however this might be considered a breaking change and should probably be discussed.

And because XMP-Toolkit-sdk handles aliases for exiv2 now, we have to keep track of every language we set for a langAlt property and overwrite them whenever necessary.

An alternative array of simple text items. Language alternatives facilitate the selection of a simple text item
based on a desired language. Each array item shall have an xml:lang qualifier. Each xml:lang value shall be
unique among the items.

XmpSpecificationPart1 page 30