Check if open projects are well formed

[vitorsouza]

The project tree model expects the project to have a specific format, but it doesn't check if it's well formed, leading to Eclipse exception in case it's not. Implement this verification.

Some things that should be verified (is there anything else?):

• Standard project folder (model, source, etc. see appropriate constants) should exist;
• Members of existing project folders (IFolders have a set of IFiles as members()) should also exist;
• Ecore models have correct references to base files [1].

See TODO note in class it.unitn.disi.unagi.rcpapp.views.models.ProjectProjectTreeElement

[1] Requirements models are created referring to the gore.ecore file that is inside the Unagi Application plug-in, resulting in a reference similar to bundleentry://1411.fwk1709572287/META-INF/gore.ecore. When there are changes in this plug-in, the generated ID changes, making all existing models invalid for code generation and rule compilation.