Blender 3.0 will only support single-frame Actions in the pose library. The goal of this patch is to lay the groundwork for making it possible for the Asset Browser to reject/hide "animation snippet" Action assets. Determining whether an Action has one or more frames (i.e. whether it has a single pose or animation) requires inspecting the Action itself, and thus loading the data-block itself. This would make it impossible to quickly determine from the asset browser. To solve this, the Action is inspected before saving, and a `"is_single_frame"` boolean (well, 0/1 integer) IDProperty is added. Reviewed by: Severin Differential Revision: https://developer.blender.org/D13202