Expo: Remove close button from window thumbnail, close window on thumbnail middle-click

This patch removes the close button that would appear on a window thumbnail when hovered. Instead, it lets you middle-click a window thumbnail to close a window.

Personally, I don't much care about this. I did the basic implementation of the window-close button with the intention that it would be refined in subsequent iterations, but that did not happen. So I'm ready to scrap the idea and move in another direction.