#38 Not a good validation at open

[Adrian Iriciuc]

When you open a new file the application does not validate the input.

Steps to reproduce:
1. Create a nea diagram
2. Add to classes and rename them class1 and class2
3. Save the diagram on the disk
4. Open the saved file with a text editor
5. Replace all occurences of "class2" with "class1"
6. In the editor opend the file

Result: two classes with the same name
Expected result: a validation error

[Adrian Iriciuc]