Add a global validation mechanism

Sometimes there is need to do some global eclipse-like validation of all resources defined in a studio project, for instance:

• when removing or changing a field in a schema, layouts referencing it should be validated to show an error
• when designing a content view layout, referencing a field that is not marked as "prefetch", the layout should be validated to show a warning

This sort of validation should be done whenever something is changed, the errors and warnings should be visible on the left tree as well as on the pages themselves, with an appropriate icon and explanation message.

Even if there are errors in the global validation, the jar export should be available.