This commit adds a new read-only boolean property `Screen.is_scrubbing`. The related property `Screen.is_animation_playing` is set to `True` in two situations: - Animation is actually playing (for example via the Play button in the timeline) - The user is scrubbing through time (in the timeline, dopesheet, graph editor, etc.) To distinguish between these two cases, the property `Screen.is_scrubbing` has been added. Concept approved by @brecht.