Preview the workspace when hovering the corresponging workspace drawing on the workspace-switcher applet

[anaximeno]

The idea is to show a workspace preview similar to how we show for the app instances when hovering the corresponding app icon in the window list applets.

As a basic wireframing, the preview could look like this:

With the blue inner rectangle being the image of the workspace.

It should also have a configuration setting to enable and disable this feature and return to the normal tooltip message on hover.