Expose YaruWindow API

[jpnurmi]

This allows apps to set the window title, and the relevant window attributes (minimizable, maximizable, closable) in a way that doesn't need the recently added updateState() workaround.

Before:

windowManager.setClosable(false);
YaruWindow.of(context).updateState(); // or YaruWindow.updateState(context);

After:

YaruWindow.of(context).setClosable(false); // or YaruWindow.setClosable(context, false);

[jpnurmi]

@Feichtmeier I promise this is the last one I need for the installer. :)

• Use YaruWindowTitleBar canonical/ubuntu-desktop-installer#1329

[Feichtmeier]

lgtm