Blender Git Loki

Tiny usability fix:

Until now, pressing ALT-A in a sequencer space without image preview was
pretty useless: It only showed an advancing green frame pointer :)

Now, if you press ALT-A over a sequencer space with strips, it will also
animate all sequence previews. So, no need to press SHIFT-ALT-A in that
situation anymore.

Of course, pressing ALT-A over a preview will still only play that preview.

Windows other than sequencer windows remain unaffected by this commit.