Improve refreshing of the project tree

[vitorsouza]

Currently, whenever the workspace change, the project tree is completely refreshed, which results in it going back to showing only one level down from projects (i.e., projects and folders), no matter what was open before the change.

Improve this behavior by using deltas in method resourceChanged() of class it.unitn.disi.unagi.rcpapp.views.UnagiProjectExplorerView. The IResourceChangeEvent event that is given as argument for this method has a method getDelta(), whose documentation should be studied for this.