Add floating layout

[Ottatop]

The idea is simple:

1. Add a "pseudo-floating" state to windows to denote windows that are supposed to be tiled but due to a lower-than-requested amount of geometries from a layout generator are floating
2. Add a layout generator that does literally nothing
3. ???
4. Profit

[ildar]

workaround, just FTR:

    Window.add_window_rule(function(window)
        window:set_floating(true)
        end)

[Ottatop]

Implemented in #326