Another step towards removing `AssetHandle`, see #108806.
When introducing the asset shelf, much of it was using the
hacky/temporary `AssetHandle` type since `AssetRepresentation` wasn't
sufficient yet. Passing an asset through context also required using
`AssetHandle`. All this has changed since then, and we can switch the
asset shelf to pass the asset through context via `AssetRepresentation`
instead, as it should be. I think none of the asset shelf uses the asset
handle anymore now, hooray :)
Minor compatibility breakage: In Python `context.active_file` doesn't
work in the asset shelf anymore now. `context.asset` can be used
instead.
7cd26d37ea