Due to lots of cases which were added and added to tree_move(), the function
was not really easy to understand. For this refactoring, I wrote tree_move()
from scratch, thinking about (hopefully) all cases. The testsuite still passes.
The move command also has different parameters now. Instead of the hard to
understand 'before v' stuff, we use 'move [left|right|up|down]'.
The file is now created in /tmp using the process PID and the
username of the user running i3. The restart state file is only
loaded when restarting (the --restart option is appended to the
command line prior to the restart). That means that renaming the
old state file with the ".old" extension is no longer needed.
This "--restart" switch is supposed to be only used by i3. The
"-L" switch can be used to load a layout (and not delete it
afterwards). We unlink the state file after we load it so that
we don't keep cruft in /tmp or try to restart from an old config
file if restart_state is set.
