Support setting client window state

Client windows here are the desktop environment windows containing the
browser viewport.

This adds two commands:

* browser.getClientWindows that returns a list of all client windows
* browser.setClientWindowState that sets the
state (fullscreen/normal/minimized/maximized) of a specific client
window, and also supports resizing of a specific client window.

It also adds the client window id when getting browsing context info.