Touch friendly changes for the Overview effect

* Makes the close button on thumbnails always visible: besides making it usable on touch, makes it more usable with mouse as well
* add a drag down gesture to close windows, moving opacity to zero while dragigng down to signal the window will be destroyed
* restore behavior animations: animate changes in thumbnails size or positions when not in drag mode or drag gesture to not make jarring sudden changes after closing a window or at end of drag mode