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/generalsettings.cpp

@@ -59,7 +59,7 @@ GeneralSettings::GeneralSettings():
 
 QString GeneralSettings::id() const
 {
-    return QLatin1String("A.General");
+    return QLatin1String(Core::Constants::SETTINGS_ID_ENVIRONMENT);
 }
 
 QString GeneralSettings::displayName() const