Adds a menu to choose the asset library to use to the asset shelf
catalogs selector popover. This is consistent with the asset browser.
After talking to animators and some artists, this is likely something
they would want to have, since they don't always want all their assets
and catalogs to populate the asset shelf. It's also necessary for the
pose library UI to provide the same features roughly than the old one
(in the sidebar, before the asset shelf was introduced).
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