Consider supporting alternate content blocks at root of part content

If a part has an mc:AlternateContent block at the root of the document, we do not load it. This is loaded fine in Office, so we may want to enable this scenario.

[twsouthwick]

That’s the beautify of the “Features” infrastructure - things can be opted in to for experimental stuff. Once #1294 is completed, it will be much easier (currently much of this stuff is handled before we return control to the user)

[tomjebo]

excerpt from internal discussion on the originating bug:

…if you add a sldMaster child AlternateContent block wrapping a transition element per [MS-PPTX] 3.1 you can get the XML that we saw in the customer’s bug. Depending on whether you call AppendChild() or how you call AddChild(), it might add that AC block at the end or the beginning or anywhere in the list of children because it does not know what order to place an AlternateContent element. Only a Transition element and we don’t dig into AC blocks when adding them. This is also something we could add as rich features to the SDK.