Blender Git Loki

Fix T92113: On assets, replace "Fake User" button with "Clear Asset" button.

Change is simple enough, but we abuse a bit the UI code here to get a
similar 'look' as the fake user button for the new Asset one, while
still being able to call an operator instead of editing directly a RNA
value.

Reviewed By: Severin, sybren

Maniphest Tasks: T92113

Differential Revision: https://developer.blender.org/D12839