refactor tree_move() into src/move.c, change config (!), change testcase

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]'.

src/cmdparse.l

@@ -112,8 +112,6 @@ up                              { return TOK_UP; }
 down                            { return TOK_DOWN; }
 left                            { return TOK_LEFT; }
 right                           { return TOK_RIGHT; }
-before                          { return TOK_BEFORE; }
-after                           { return TOK_AFTER; }
 resize                          { return TOK_RESIZE; }
 shrink                          { return TOK_SHRINK; }
 grow                            { return TOK_GROW; }