Improving the snippets on AppBar class doc

[jayoung-lee]

AppBar/actions doc on master docs has 'Sample in an App' tab, but the current example is not really complete.

The 'actions' keyword imply interactivity, but because its onPressed (){} property is empty, at least 3 participants in the recent user study spent long time searching for its example, even though they had this snippet.

Can we make it extend StatefulWidget like the one in IconButton class and have an actual action associated with it?

This complete app can be added to the AppBar class as well, which is currently empty!

Cc: @galeyang, @gspencergoog

[jayoung-lee]

Moved to flutter/flutter repo (here)