Fix occurrences of the contains/insert anti-pattern

Introduce and make use of Utils::insert() for QSet with a return value
that indicates whether insertion actually happened.

Change-Id: I655e4bc3553b74fea5ae8956205e4d8070118d63
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/algorithm.h

@@ -1437,4 +1437,13 @@ void addToHash(QHash<Key, T> *result, const QHash<Key, T> &additionalContents)
     result->insert(additionalContents);
 }
 
+// Workaround for missing information from QSet::insert()
+// Return type could be a pair like for std::set, but we never use the iterator anyway.
+template<typename T, typename U> [[nodiscard]] bool insert(QSet<T> &s, const U &v)
+{
+    const int oldSize = s.size();
+    s.insert(v);
+    return s.size() > oldSize;
+}
+
 } // namespace Utils