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/plugins/qmljseditor/qmljscompletionassist.cpp

@@ -16,6 +16,7 @@
 #include <texteditor/texteditorsettings.h>
 #include <texteditor/completionsettings.h>
 
+#include <utils/algorithm.h>
 #include <utils/qtcassert.h>
 
 #include <qmljs/qmljsmodelmanagerinterface.h>
@@ -261,10 +262,9 @@ private:
 
     void processProperties(const ObjectValue *object)
     {
-        if (! object || _processed.contains(object))
+        if (! object || !Utils::insert(_processed, object))
             return;
 
-        _processed.insert(object);
         processProperties(object->prototype(_scopeChain->context()));
 
         _currentObject = object;