work on the `chrome.app.window` API + some basic window tests
restoring `ChromeApp` and `ChromeWindow` as public types with private ctors
dartdoc the AppWindow

more or less a copy / paste from the javascript API docs with small
style fix (dart style guide uses 'this' instead of 'the' on instance
method docs)
put the dartdoc back on the `current` getter
more dartdoc
more dartdoc