Add `PointerRNA::reset()` and `PointerRNA::invalidate()` utils functions
(both simply reset the PointerRNA data to empty state).
Replace `RNA_POINTER_INVALIDATE` macro by `PointerRNA::invalidate()`.
Follow-up to !134393 and e55d478c64.
While the python API for the legacy Grease Pencil object can be fully
deprecated, annotations still make use of the `bGPdata` DNA and
therefore need their own python API.
This removes `RNA_def_gpencil` entirely, but replaces parts of it
in `RNA_def_annotations` to make sure that annotations keep
working as before. Note that this is only a very small subset of the
legacy Grease Pencil API.
Technically, this "removes" python API functionality from annotations
because they are full Grease Pencil IDs, but the idea is to make them
simpler and encourage users to use Grease Pencil objects instead.
Pull Request: https://projects.blender.org/blender/blender/pulls/125097
