Better integration of XWiki Eclipse with RCP Applications and the Eclipse ecosystem

In order to provide a better integration of the XWiki Eclipse plugin two changes could be made:

1) Add the org.xwiki.plugins.eclipse.model to the exported packages of MANIFEST.MF. In this way XWiki Eclipse domain model's interfaces would be available to other plugins to make talk them directly with.

2) Publish XWikiNavigator selection. This is a way for other plugins to retrieve the current selection in a loosely coupled way.