Add fields and methods to Classes

The Dart Designer Class Diagram now has support for:
- fields (with direct edit)
- methods (with direct edit)
- parameters (with direct edit)
- compositions, imports and exports

Bug-Url: https://github.com/dartdesigner/Dart-Designer/issues/21
Change-Id: Ia1fc3fabfc41cd65427081a9f767937fd419fb7e
Add a link to the Dart Programming Language Specification

Change-Id: I25d60660a965d8bcaac7b42dbe927539c97e3742