Re-add monocle mode

I often find myself using "Mod+f" to set a window to full screen. I
often use this because the terminal is not wide enough to show certain
output (mysql tables, logfiles, ...). This technique works ... fine.

There's several problems, though:

 1. My status bar gets hidden.
 2. If I open another window, it gets placed *behind* the window in full
    screen. Plus, the new window has focus. This is very annoying.

Let's see how monocle mode works.

This reverts commit 27500f41d099b94c0f48058db018d3ec8afe6269.