++++++++++++++++++
- add PicturePlaceholder with .insert_picture() method
- add TablePlaceholder with .insert_table() method
- add ChartPlaceholder with .insert_chart() method
- add Picture.image property, returning Image object
- add Picture.crop_left, .crop_top, .crop_right, and .crop_bottom
- add Shape.placeholder_format and PlaceholderFormat object
**BACKWARD INCOMPATIBILITIES:**
Shape.shape_type is now unconditionally `MSO_SHAPE_TYPE.PLACEHOLDER` for all
placeholder shapes. Previously, some placeholder shapes reported
`MSO_SHAPE_TYPE.AUTO_SHAPE`, `MSO_SHAPE_TYPE.CHART`,
`MSO_SHAPE_TYPE.PICTURE`, or `MSO_SHAPE_TYPE.TABLE` for that property.