Fix #421

Fix #421 by always putting floating windows above the Max layout (that already put windows above).

The problem with this is that it does not respect floating windows z-order if modified (the order will always stay the same, by order of floating window appearance). I think it does not matter that much since I don't see any other usage of floating windows in Max mode other than for dialogs.

A better way would be to make the above parameter for `window.place()` acting only for current layout, or having a way to handle z-index between layouts (i.e. floating layout have is own z-index but each one of its windows will always be above the current layout), but it seems to me that it would require a lot of work on top of XCB.