<else> tag is supported for <with>
getSceneById() is used at <goto> command

less copypaste, more code reuse!
<with> and <choice> now can be nested

Check out some test samples to get the real power of nesting!
<when>, <else> and <option> now can contain just the same tags as
<scene> itself!
Interpreter stack is used for managing nesting, note that it is pop'd
when scene is actually changed.
fix of var replacement at <var> and better sample for nested <with>