ability to show only spaces with windows on them

[edgimar]

Same issue as reported here:

It would be convenient to show only spaces that have windows assigned to them, instead of showing all spaces, even when they are completely empty. For example, something like:

[2][4][5]

instead of
[1][2][3][4][5]

(assuming spaces 1 and 3 do not have any windows on them)