Blender Git Loki

Assets: Clear Asset operator variants for clearing/setting Fake User

The Clear Asset operator (`ASSET_OT_clear`) now clears the Fake User.
This makes it symmetrical with the Mark Asset (`ASSET_OT_mark`)
operator, which sets Fake User to ensure assets are always saved to
disk.

Clear Asset now also has a `set_fake_user` boolean option, which allows
users to Clear Asset and set Fake User in one go.

The asset browser now shows these options in the context menu:
- Clear Asset: also clears Fake User. This makes it possible to actually
remove assets from the blend file without leaving the Asset Browser.
- Clear Asset (Set Fake User): keeps the Fake User bit set. This makes
it possible to "hide" the asset from the asset browser, without
loosing the actual data.

Internally, the `ED_asset_clear_id(id)` function now always clears the
Fake User bit. If it was intended that this bit was kept set, it's up to
the caller to explicitly call `id_fake_user_set(id)` afterwards.

Manifest Task: T90844

Reviewed By: Severin

Differential Revision: https://developer.blender.org/D12663