core/window: add parentWindow property to FloatingWindow

[bbedward]

Allows setting a parentWindow for FloatingWindow , which on wayland calls xdg_toplevel::set_parent .

Compositors may handle child windows differently (e.g. a dialog), by opening them as floating / positioned within the window that triggered them. This allows creating windows with that behavior.

[outfoxxed]

Looks good except this case which does not work

FloatingWindow {
	id: w

	FloatingWindow {
		parentWindow: w
		visible: true
	}
}

if initially invisible and later made visible it works as expected

[bbedward]

Looks good except this case which does not work

FloatingWindow {
	id: w

	FloatingWindow {
		parentWindow: w
		visible: true
	}
}

if initially invisible and later made visible it works as expected

I changed to hook into visibility and that case is now fixed