This makes the asset shelf available as a standard, non-experimental
feature. Currently no script makes use of it, so it will still not be
available. However the pose library will be updated to use this for its
UI instead.
Initially the asset shelf was committed as experimental feature, even
though it seemed ready for non-experimental. After double checking with
some people (Dalai and Brecht) we decided to go ahead with this.
There are still some futher changes planned, see #107881.
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