gnome: Run gtkdoc-scanobjs and add a way to get assets working

Allowing the object tree to be generated.

We need to add options to allow copying the ncesseary sources and
assets so the HTML generator can work with them (everything is
relative so we need to copy them in the build directory).

Until now the documentation was not generated from the user provided
main sgml file but it was using a generated one, which lead to a broken
documentation. Starting using it revealed the other bugs fixed in that
commit.

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