Essentially move the Object-handling logic also into
`bke::greasepencil::convert::`. This code will also be needed for
automatic conversion on fileread etc.
It also helps to keep all the conversion logic in one place (especially
since there is going to be way more done at object level - modifiers,
animation, etc.).
Pull Request: https://projects.blender.org/blender/blender/pulls/118384
Bastien Montagne
9275d40934