Importing / closed projects throw a lot of errors

Hello everyone,

I noticed that when importing projects, there are a lot of errors like this in the `Error Log` view:

> isGWTProject(project) hasGwtnature exception Project 'CertificateManager' is not open.

It seems to originate from [HERE](https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/v4.2.0/plugins/com.gwtplugins.gwt.eclipse.core/src/com/google/gwt/eclipse/core/nature/GWTNature.java#L53-L57). I guess that the [ResourceChangeListener](https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/v4.2.0/plugins/com.gwtplugins.gdt.eclipse.core/src/com/google/gdt/eclipse/core/reference/ResourceChangeListener.java) might be implemented incorrectly, assuming all projects are opened, which is then triggering the initial issue in `GWTNature#isGwtProject`.

When importing projects, we do that automatically, and initially, they are always closed before being opened. We don't have a special implementation; we rely on the Eclipse API.

Once everything is finished and the projects are all imported (and open), there are no further errors by the GWT plug-in, and everything works as expected. But this is quite annoying nevertheless.

Best,
Tobias

plugins/com.gwtplugins.gwt.eclipse.core/src/com/google/gwt/eclipse/core/nature/GWTNature.java の GWTNature#isGwtProject と、plugins/com.gwtplugins.gdt.eclipse.core/src/com/google/gdt/eclipse/core/reference/ResourceChangeListener.java の ResourceChangeListener から始めます。プロジェクトを閉じた状態でインポートを再現し、その後リソース変更の処理を追跡します。完了の条件は、閉じたプロジェクトをインポートしても報告されたエラーが発生せず、通常のプロジェクト検出が引き続き機能することです。

索引モデルが issue の本文から書いたものです。