Refactor the interface of commands.c

This change has two implications:

1) tree_render() will now be called precisely once for input which consists of
   multiple commands (like "focus left; focus right"). Also, the caller of
   parse_command() has to call it. This makes us able to fix tickets such as
   ticket #608 (where multiple tree_render() calls are noticable).

2) The output of a command is now a JSON array of return values of the
   individual subcommands. In the case of "focus left; focus right", this is:

   [{"success":true}, {"success":true}]

   While this is incompatible with what i3 returned before, the return value of
   commands was undocumented and therefore not subject to our API stability.

testcases/lib/i3test.pm

@@ -247,7 +247,7 @@ sub open_empty_con {
     my ($i3) = @_;
 
     my $reply = $i3->command('open')->recv;
-    return $reply->{id};
+    return $reply->[0]->{id};
 }
 
 sub get_workspace_names {