Unions make handling of non-trivial data 'members'... non-trivial.
Since `PointerRNA` is going to become non-trivial, this commit replaces
the Union storing delta, min and max values of editied RNA properties
in `ui_selectcontext_apply`, into a `std::variant`.
It also adds a new accessor to RNA string properties, returning a
`std::string` data, and uses this type to store string properties values
in `ui_selectcontext_apply`.
Pull Request: https://projects.blender.org/blender/blender/pulls/124727
50076993f3