Various UI code would store the `AssetHandle` in a way that turns out to
be unsafe. The file-data is part of the file browser caching system that
releases file-data when a certain maximum of items is in the cache. So
even while just iterating over the assets, earlier iterated asset
handles may become invalid. Some
For the asset-view, the fix was more involved. There we need an RNA
collection of asset-handles, because the UI list code requires that. So
we create a dummy collection and get the asset handles as needed by
index. This again meant that I had to keep the index of the collection
and the asset-list in sync, so all filtering had to be moved to the UI
list.
I tried duplicating the file-data out of the cache instead, but that
caused problems with managing the memory/ownership of the preview
images.
`AssetHandle` should be removed and replaced by `AssetRepresentation`,
but this would be an even more disruptive change (breaking API
compatibility too).
Also fixes#105535.
