Support setting client window state (#752)

* 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.

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Co-authored-by: Henrik Skupin <[email protected]>
Co-authored-by: Maksim Sadym <[email protected]>