Blender Git Loki

Fix T52331: Motion blur shutter length not keyable

The only reason shutter time was marked as non-animatable is because Blender
Internal render does not support such animation. But this is something what
users are keeping asking for and now Blender Internal is on it's way out.

Enabled animation of this property, but noted in tooltip that Blender Internal
does not support animation of this property.