Foldernavigationwidget: Add "Open Terminal" and "Open Explorer".

Move code to Foldernavigationwidget as static utilities and use
them from there and from the Project explorer. Fix error message box.
Task-number: QTCREATORBUG-62

src/plugins/coreplugin/coreconstants.h

@@ -225,6 +225,7 @@ const char * const WIZARD_TR_CATEGORY_QT = QT_TRANSLATE_NOOP("Core", "Qt");
 
 const char * const SETTINGS_CATEGORY_CORE = "A.Core";
 const char * const SETTINGS_TR_CATEGORY_CORE = QT_TRANSLATE_NOOP("Core", "Environment");
+const char * const SETTINGS_ID_ENVIRONMENT = "A.General";
 
 } // namespace Constants
 } // namespace Core