Enable XML Text Entities

[alfsb]

This is the final steps into enabling XML "text" entities, to be used in parallel with actual DTD macros.

Plan to merge this in one week.

There may be a few renames. and after merging, I may open new PRs, first demonstrating the new funcionality, and even trying to pass some mass convertions.

The end game objective is to eliminate all .ent files from doc-base and doc-en, replacing it with global.ent, manual.ent and remove.ent XML files, and also enabling individual entity files, stored as doc-lang/entities/name.xml.

Comments and reviews wellcome.

[jordikroon]

Just a few nits, but functionally this looks good to me and it's a good way forward.
Thank you for these changes.

[alfsb]

@Girgias , I think it may possible to have extension entities on the first version. Should this per extension entities files be placed as:

1. doc-lang/entities/extname.ent
2. doc-lang/reference/extname/entities.ent
3. Anywhere below doc-lang/entities/ and doc-lang/reference/ ?

[jordikroon]

I think the only correct place would be within the extension folder. So doc-lang/reference/extname/entities.ent.
Similar to versions.xml you want everything related to an extension to be located in the same folder.

[alfsb]

Sorry for the big rewriting commit. It's easier to inspect each final file than the diffs. I'm trying to keep the original merge window for now. Individual (.xml) and grouped (.ent) XML entities is ok and tested. Grouped XML entities on reference/ is still missing, but the code is prepared for this.