Remove z-indexes from components

[mikewheaton]

Many of the components have z-index values baked in, which might work relative to other Fabric components but are often wrong for fitting into an existing app. I haven't dug into the Layer component but I understand it is an alternative to this. If not, we should at least provide an interface to every z-index so that developers can stack the components however their app requires.

[dzearing]

thanks for adding this!

[dzearing]

I think for the most part this has been done.