Fix focusing a screen when the screen has no windows.

[Lukas-Stuehrk]

I'm unsure about the naming of the cgRect method. Please feel free to suggest a better name.

Resolves #533

[ianyh]

The naming seems fine for now, but I tested this and it doesn't seem to solve the problem. That function never gets called. It looks like something in this function is failing to focus the screen when there are no windows. https://github.com/ianyh/Amethyst/blob/development/Amethyst/Managers/WindowModifier.swift#L287

[Lukas-Stuehrk]

Interesting....gets called for me at https://github.com/ianyh/Amethyst/blob/development/Amethyst/Managers/WindowModifier.swift#L307 in exactly this method.

macOS 10.13.6 (17G65) with neither mff nor ffm activated.

[ianyh]

So I think it's actually this line https://github.com/ianyh/Amethyst/blob/development/Amethyst/Managers/WindowModifier.swift#L293

There is a focused window, but it's on a different screen. I seem to hit that condition when trying to focus a screen with no windows. I'm not sure why it's only happening to me on your branch.