Add Screen.cmd_togglegroup()

cmd_togglegroup() switches to the previously selected group and back.

I find it very useful to quickly display a group and then move back without thinking where I was originally, or if my workflow involves continuously switching between two groups.

If a group name is specified, cmd_togglegroup() will move there unless it is already the selected group: in this case, it will switch back to the previously active one.

If no arguments are specified, cmd_togglegroup() will just move back and forth between the actual and the previous group.


The pull also moves cmd_groupnext() and cmd_groupprev() from Group to Screen.
It is the first time I fiddle with qtile and I should have done this in two different commits.
It is also the first time I try to contribute to to a github project, I hope you find it useful.