UIs depending on assets must handle invalid assets

When creating the `pom.xml` editor, a big discussion has emerged on this PR.

For example, the Project Settings screen depends heavily on three files:
1. `pom.xml`
2. `kie-deployments-descriptor.xml`
3. `persistence.xml`

It's possible to edit the contents of these files using the default XML editor, which can be opened through the Project Explorer.

Screens like this must adapt to the fact that the assets it depends on might be at an invalid state and provide ways for the user to 'fix' the asset data. One idea for this particular example is to open the XML editor and let the user modify and save the asset. After closing the asset, the user should go back to the original screen he was trying to access on the first place.

A similar issue happens on the Library screen because it depends on the `pom.xml` file.