++++++++++++++++++
- Add `Picture.crop_x` setters, allowing picture cropping values to be set,
in addition to interrogated.
- Add `Slide.background` and `SlideMaster.background`, allowing the
background fill to be set for an individual slide or for all slides based
on a slide master.
- Add option `shapes` parameter to `Shapes.add_group_shape`, allowing a group
shape to be formed from a number of existing shapes.
- Improve efficiency of `Shapes._next_shape_id` property to improve
performance on high shape-count slides.