Can't set title to empty string

[1j01]

Just noticed this deficiency: https://github.com/1j01/os-gui/blob/bb4df0f0c26969c089858118130975cd137cdac8/%24Window.js#L1480

It's a jQuery-style method that either sets or gets the title, but empty strings are falsy so this would be a slight pitfall as it would simply return the current title instead of setting it to the empty string.