Asset System: New Asset Browser editor

This introduces the User Interface part of the Asset Browser, based on the
design in T54642.

Additions:
* New Asset Browser (internally a sub-editor of the File Browser).
* Navigation region showing asset categories.
* Main region showing the assets of the selected asset library with previews.
  The assets may be stored over multiple .blends in the directory that's
  "mounted" as asset library in the Preferences. They will all be shown in this
  list.
* Header with an asset library dropdown, allowing to choose the active asset
  library to show. Options are the "Current File" as asset library and all
  custom libraries.
* Display popover, filter popover and search box (partially dummies, see
  T82680).
* Sidebar showing the metadata of the currently active file (name, preview,
  description and tags), which can be edited for assets in the "Current File"
  asset library. (For others it will reset on reload.)
* The sidebar includes a button to load a custom preview image from a file.
* Make asset files draggable (with preview image).
* If a library with invalid path is selected, a message is drawn in the main
  region to help the user understand what's wrong.
* Operators to add and remove asset tags. Exposed in the sidebar.
* "Only Assets" option for Link/Append.
* Internal utilities for asset UI scripts.

For screenshots or demo videos, please see D9725. Or the 2.92 release notes.

Note that there are many things to be tweaked and polished in the Asset Browser
UI still. For example, the filter and display popovers are mostly dummies. See
T82680.

Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.

Differential Revision: https://developer.blender.org/D9725

Reviewed by: Brecht Van Lommel, Hans Goudey

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