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.
2be7a11e43