CONFIG BREAK: kick the 'next'/'prev' commands, introduce 'focus left' etc.

Instead of the cryptic next/prev commands, we now provide an argument to the
focus command. Userguide and default config is updated.

src/cmdparse.l

@@ -120,7 +120,6 @@ workspace                       { WS_STRING; return TOK_WORKSPACE; }
 focus                           { return TOK_FOCUS; }
 move                            { return TOK_MOVE; }
 open                            { return TOK_OPEN; }
-next                            { return TOK_NEXT; }
 prev                            { return TOK_PREV; }
 split                           { return TOK_SPLIT; }
 horizontal                      { return TOK_HORIZONTAL; }