Blender Git Loki

Asset System: Prepare File Browser backend for the Asset Browser

The Asset Browser will be a sub-editor of the File Browser. This prepares the
File Browser code for that.

**File-Lists**
* Support loading assets with metadata read from external files into the
file-list.
* New main based file-list type, for the "Current File" asset library.
* Refresh file-list when switching between browse modes or asset libraries.
* Support empty file-lists (asset library with no assets).
* Store file previews as icons, so scripts can reference them via icon-id. See
previous commit.

**Space Data**
* Introduce "browse mode" to differeniate between file and asset browsing.
* Add `FileAssetSelectParams` to `SpaceFile`, with `FileSelectParams` as base.
Makes sure data is separated between asset and file browsing when switching
between them. The active params can be obtained through
`ED_fileselect_get_active_params()`.
* `FileAssetSelectParams` stores the currently visible asset library ID.
* Introduce file history abstraction so file and asset browsing can keep a
separate history (previous and next directories).

**General**
* Option to only show asset data-blocks while file browsing (not exposed here).
* Add "active_file" context member, so scripts can get and display info about
the active file.
* Add "active_id" context member, so `ED_OT_lib_id_load_custom_preview` can set
a custom ID preview. (Only for "Current File" asset library)
* Expose some of `FileDirEntry` in RNA as (non-editable). That way scripts can
obtain name, preview icon and asset-data.

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/D9724

Reviewed by: Bastien Montagne