Switch to xcb-xkb and libxkbcommon

This removes our last dependency on Xlib! :)

(Okay, an Xlib dependency still comes in through other libraries that we
 link against, but it’s not us. Our code is simpler by this change and
 uses one less connection to X11.)
DEPENDS: add libxkbcommon{,-x11}, update lkgv (Thanks okraits)
Consistently parse workspace numbers

Use ws_name_to_number() to parse workspace numbers where this
transformation takes place.
Make command `move [direction]` work with criteria

A container selected with criteria should be moved with the `move
[direction]` command, instead of this command always acting on the
focused container.
Implement the window::floating event

The window::floating event should be emitted when a window transitions
to or from the floating state.