Some improvements to Desktop Preview mode

[somebody2978]

I thinks than these can be a good idea to Desktop Preview mode:

- A Auto-fade transparency like alternative Auto-hide to Desktop Preview. To avoid the animation.
- A Look position, sometimes moving icons here, accidentally move the position.
- Possibility to remove the text background (Desktop 1, Desktop 2, Desktop 3, Desktop 4 texts) of each desktop box. Sometimes that text hinders the visualization the icons correctly.
- When using do icon appearance, create on the window icon a thumbnail of the contents automatically when the mouse pointer is resting on this icon.

That's it, I hope to help.