Blender Git Loki

3D Annotations: Restore ability to access annotations from the 3D view

For now, the "gpencil" context properties work as follows:
* If the active object is a GP Object, the GP data associated with
that is returned
* Otherwise, the Scene's GP data (i.e. annotations) are used

We may need to revise this later. But in the meantime, I'm restoring
this so that we have a quick way of figuring out what parts of the
annotation tool workflow are currently broken.