follow_master_background is not settable

[MartinPacker]

For a Slide object follow_master_background is not settable - despite the docs saying it should be.

My use case is adding a "pseudo background" i.e. a shape at the back of the z order that covers the whole slide. (My md2pptx users would be able to set a custom background per page with my code.)

It seemed to me tidy to set follow_master_background to False under these circumstances.