Allow hiding empty desktops on desktops widget

[Eloi]

Desktops widget takes too much screen real estate when using multiple monitors with multiple desktops.

Per example, I usually use up to three monitors with 12 desktops in each one, for a total of 36 desktops, which takes half of my monitor only for the desktops widget.

In some WM (like Awesome) there is an option to hide empty desktops, and only show them when they have something contained or when they are selected.

This is specific to my workflow, but I think that its general enough to be useful for everyone, so I implemented a config option to allow hiding empty desktops and packed this feature in a pull request.