Comment 4 for bug 842578

This'll be a fun one to implement. Some design thoughts that probably need refining first:

  * Aspect ratio. Should the aspect ratio of the display be reflected in the Show Desktop icon
  * How should the aspect of the wallpaper be applied to the aspect of the preview icon. Is there a maximum constraint (eg. very wide wallpaper screeched across multiple monitors with ratio of 64:9)
  * If there are multiple monitors; should the icons (or previews) be different ratios on different monitors
  * If the user is running a desktop-wallpaper cycler in the background, should that the icon be updated in real-time
  * If the user rotates their device (so changing the aspect ratio) whilst the Alt-tab is open, should that be reflect. Should there be a transition