Add the property API

[jecaro]

This PR adds part of the property API of taglib.

The properties are basically a key/value map. The keys are strings and the values are list of strings. To keep up with the original design, one must create a getter for a specific property.

All the functions of the property API have been ported, but this one, as I don't need it:

/*!
 * Get the keys of the property map.
 *
 * \return NULL terminated array of C-strings (char *), only NULL if empty.
 * It must be freed by the client using taglib_property_free().
 */
TAGLIB_C_EXPORT char** taglib_property_keys(const TagLib_File *file);

But I can add it as part of this PR if you think that'd be needed to merge it.

For the context, I'm building a CLI tagger based on htaglib: htagcli. I need to access the albumartist tag, which is not exposed in the simple API. It is really a shame, as it is an important tag for organizing audio files.

[jecaro]

Looks like this property API is not available in taglib 1.x, the version shipped with ubuntu-latest, which is 24.04, and taglib 2.x is not available on this Ubuntu version 😢

I guess we're good to wait for next Ubuntu LTS next year: 26.04

[jecaro]

I just tried this branch in my project, and it crashes.

Marking as draft, I guess it's not quite ready yet anyway 😅

[jecaro]

Ok. That one is fixed with 13d5981

I'll start using this branch in my project.

I leave it as a draft for now, as there is still the issue with Ubuntu anyway.

Once I find it stable enough for my project, I'll remove the draft flag.

[mrkkrp]

Hey, thanks for this PR! Let me know when you deem it ready.