QIcon: ensure that QIconLoader is initialized

Make sure that QIconLoader is initialized when QIcon::themeName()
is called. Otherwise, themeName() may return the wrong value.

Change-Id: I54621e31da24bdbe745e7d6ae6a1d58cb4190cd7
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/gui/image/qicon.cpp

@@ -945,6 +945,7 @@ void QIcon::setThemeName(const QString &name)
 */
 QString QIcon::themeName()
 {
+    QIconLoader::instance()->ensureInitialized();
     return QIconLoader::instance()->themeName();
 }