Ability to pull slide information

#### Description:

@jamesramsden-bh suggested that it should be possible to retrieve element names from the template presentation, and I add that it would also be useful to be able to get all the information from the slide masters as well (for use in conjunction with the updates to be made in #16).

Three new objects, `SlideInfo`, `SlideMasterInfo` and `SlideLayoutInfo` that could be requested via the adapter would be useful for both updating and creating slides for user experience, as a user would no longer have to dig through the template presentation to know what element names to use for a slide.