Always set tiling_floating.

This is untested, as it isn't even hooked up to anything yet.

This allows moving tiles with the keyboard.

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.

(Also removes superfluous keepBelow setting)

Fullscreen windows should be above _everything_, so keepBelow should be false.

This features full resizing.

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.

Prevents .xsession-errors from getting big.

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.

This also fixes a bug with tiling because a method was missing.