FileBrowser: Reduce Overhead Browsing Libraries.

When Browsing libraries the asset files were opened multiple times.
once to determine the needed groups to query and once for each
group to query the items in the group. For file browsing this makes sense
but for asset browsing this can be reduced.

This patch will load the asset files recursively and only opens them once.
Another change is that only the assets are requested and not filtered out
later in the process.

This patch is needed to simplify the library indexing. Where
we need access to the full library content.

## The numbers ##

Benchmarked by adding scenes of the spring open movie to the default
asset library. Refreshing the asset library would recursively load all the files
there.

| **8bc27c508a** | Processed 317 'directories/libraries' | 7.573986s |
| **Patch** | Processed 42 'directories/libraries' | 0.821013s |

{F10442811}

Reviewed By: mont29, Severin

Maniphest Tasks: T91406

Differential Revision: https://developer.blender.org/D12499

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