UI: Add asset shelf Python UI template

Adds a new "Ui Asset Shelf" template to the script editor, demonstrating
how scripts can register own asset shelves. The asset shelf is designed
as a new standard UI element that add-ons, application templates and the
like can use, so this is useful. It's also useful as a quick way to test
and customize the asset shelf.

The script is only available in the UI if the "Asset Shelf" experimental
feature is enabled in the Preferences.

scripts/templates_py/ui_asset_shelf.py

@@ -0,0 +1,26 @@
+import bpy
+
+
+class MyAssetShelf(bpy.types.AssetShelf):
+    bl_space_type = 'VIEW_3D'
+    bl_idname = "my_template.my_material_asset_shelf"
+
+    @classmethod
+    def poll(cls, context):
+        return bool(context.object and context.object.mode == 'OBJECT')
+
+    @classmethod
+    def asset_poll(cls, asset):
+        return asset.file_data.id_type in {'MATERIAL', 'OBJECT'}
+
+
+def register():
+    bpy.utils.register_class(MyAssetShelf)
+
+
+def unregister():
+    bpy.utils.unregister_class(MyAssetShelf)
+
+
+if __name__ == "__main__":
+    register()