Utils: Expand strings in StringAspect

When needed. Take a short cut on empty strings which can't have macros.

Change-Id: Icc2fb8f001da6be7a9dfec73951d9ac14d2f356f
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/utils/aspects.cpp

@@ -1290,7 +1290,8 @@ void StringAspect::addToLayout(LayoutItem &parent)
 
 QString StringAspect::expandedValue() const
 {
-    // FIXME: Use macroexpander here later.
+    if (!m_internal.isEmpty() && d->m_expanderProvider)
+        return d->m_expanderProvider()->expand(m_internal);
     return m_internal;
 }