Improve the code for the generation of Javascript code

[dmytro-grankin]

The code for the generation of Javascript was improved in SpineEventEngine/base#255.

But still there is a place for other improvements:

• Snippet.value() should return an immutable result. Now, it returns mutable CodeLines.
• CodeLines should not contain methods to compose code (enterMethod, enterBlock etc). Instead, such a functionality should be implemented by inheriting CodeLine (see RawLine) or by implementing Snippet (see Method).
• Remove JsCodeGenerators and use Snippet instead.