This only does one simple special case for the spirallayout (two clients on screen).

tiling also needs to have the type declared, because that's used to find the index.

Otherwise it'll switch once from the default and then never again.

This is much cleaner than the hodgepodge we had before
and also means e.g. desktopswitching can be handled correctly.

It is now effectively a layout.

This requires a semantics change: addClient can now also deal with already managed clients.