Expose properties for the built-in app bar

[rydmike]

To reduce the need to frequently be forced to build an entirely new custom app bar, when using the Storyboard() within a material app as a widget, also consider exposing some of properties for the built in app bar, like:

Widget title double elevation Color backgroundColor

To allow for complete custom themeing of the built in appbar also consider exposing:

IconThemeData iconTheme IconThemeData actionsIconTheme TextTheme textTheme Brightness brightness

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Alternatively, for using a custom app bar, consider showing an example that implements a custom app bar, with re-implementation of the controls on the built in one. I had a quick look at this as an alternative to get it themed the way I wanted that way. While I could get it themed the way I wanted it to look, by making a completely custom one, I could not figure out how to re-implement the controls of the built-in appbar, so that they would actually work in the custom appbar as well. Is this supported? How?