Grease Pencil py-api

* new/remove for GPencil frames/strokes/
* add/pop for points
* clear for frame/layer & grease_pencil
* copy for frame

+ fix for free_gpencil_frames() not clearing the active frame

source/blender/blenkernel/intern/gpencil.c

@@ -87,6 +87,7 @@ void free_gpencil_frames(bGPDlayer *gpl)
 		free_gpencil_strokes(gpf);
 		BLI_freelinkN(&gpl->frames, gpf);
 	}
+	gpl->actframe = NULL;
 }
 
 /* Free all of the gp-layers for a viewport (list should be &gpd->layers or so) */