Improve Thumbnail Grid Task Switcher

Currently the layout is a bit visually busy and draws your focus all
over the place. This commit makes the grid delegates use basically the
same layout as the popular window delegates in the Overview and Present
Windows effects:
- Centered window title
- Large app icon overlaid on the bottom center of the thumbnail
- close button only appears for the focused and/or hovered item

The result is better visual parse-ability, particularly for people who
prefer to navigate by app icon rather than thumbnail image.

CCBUG: 433034