Issues with Project Explorer

Whilst working with Project Explorer contributions for DDT I found an issue with Goclipse's contribution that makes it override the labels of any other plugin, even for non-Go projects, under certain circumstances (basically any Navigator Content Contribution that doesn't use the override mechanism). So for example for the latest version of DDT, if Goclipse is installed as well it won't let any labels be provided for D projects.

This is because the label provider will be applied to the possibleChildren element, regardless of trigger points. And the current Goclipse possibleChildren has a selection that applies to any resource, regardless of project nature! This pull request fixes that.

(another fix would be to return null from the label provider if the project is not of a Go nature, but this method is slightly better because it prevents Goclipse plugin from activating if a non-Goclipse project is opened)