Support diagramTextProvider priority, so new ones can override existing

[hallvard]

You may want to customise an existing diagramTextProvider, and override it. This could be supported by adding a priority attribute to the extension point schema and use a default priority for those we provide.

[hallvard]

Supported by adding a priority attribute. diagramTextProviders are normally given "normal" priority, but "default" (lower) and "custom" (higher) is also supported. Default is used by the ecore object diagram provider, since it handles eObject structures in general.