Allow custom z-index values for components

[ErinCoughlan]

Not all apps use only Angular Material components. They can use other component libraries as well as custom components, which can have different layering strategies.

By adding a z-index override attribute to components, products are able to use them without overriding the z-index on private classes.

There are also defined z-indices in Material, which all components should default to using.
Material Spec

[ThomasBurleson]

[bradrich]

This would be easily solved by our transition to mdPanel for all of the Material components. It might complicate the following of the Material spec that @ThomasBurleson added above because it would leave the positioning of items up to the developer, but it would solve several issues related to z-indexes.