support adding slide_layout

[wkpark]

WIP

This is merely one example.

It does not function perfectly, and only some features work as intended. Nevertheless, if you are planning to add or edit slide layouts, please refer to this.

• [x] add_layout()
• [x] add_placeholder()

https://github.com/scanny/python-pptx/issues/1044 - apply this suggestion into this PR.

examples

...
    prs = Presentation()
    ph_list = [
        PP_PLACEHOLDER.TITLE,
        PP_PLACEHOLDER.CENTER_TITLE
        PP_PLACEHOLDER.SUBTITLE,
        PP_PLACEHOLDER.BODY,
    ]
...
    layout= prs.slide_layouts.add_layout("My Layout %s")
...
    ph = layout.shapes.add_placeholder(PP_PLACEHOLDER.TITLE, "horz", "full")
    ph.text = "insert Title here"
...