Add SplitButton style for CommandBar

[teaP]

This change enables SplitButton to look like an AppBarButton in CommandBar, even though it isn't. I essentially copied the default SplitButton template and replaced all the colors, removed the dividing line between the two sides of the button, and adjusted the size/margins.

[mdtauk]

So it is functionally a button, but it has a chevron? Or is the chevron and button two separate hit-targets, and it behaves like a SplitButton?

If it is functionally a button, then its a DropDownButton surely, rather than a SplitButton.

[teaP]

So it is functionally a button, but it has a chevron? Or is the chevron and button two separate hit-targets, and it behaves like a SplitButton?

It's a SplitButton and behaves like a SplitButton, with two hit targets -- this is just a retemplate of SplitButton to look good in CommandBar.

[ranjeshj]

/azp run

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