Former ideas for implementing the floating of single windows in herbstluftwm.
But now it doesn't need to be implemented directly because auf the possibility
to control the monitor stacking order. The former ideas were:

  * When toggling the floating-property of a window doesn't change the position
    in the binary tree.

  * In a frame, no space is wasted for the "tiled" rectangle (i.e. position and
    size) of a floating window.

  * When navigating through the binary tree via the focus command all floating
    windows are skipped. So they only can be focused by mouse or with the
    cycle_all or cycle.

  * When shift'ing a floating window, then the layout tree changes (i.e. the
    output of layout or dump), but window rectangles (positions and sizes)
    don't change

  * Configurable: Either let all floating windows float above tiled windows or
    allow them to be between/behind tiled windows.

  * Something like: Switch focus between floating and tiling layer.