This only applies to the focus hotkeys

This is literally a one-line change.

We always seem to get the full area when we ask for a desktop that's not
the first (0).

It's better to be notified when something goes wrong than for it to be
ignored silently.

This is completely unnecessary.

Should also fix e.g. Firefox resizing.

This removes any remains of the idea of a "floating tile".
We're not a window manager, we just need to care about tiling windows.

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.