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/fakevim/fakevimplugin.cpp

@@ -45,6 +45,7 @@
 #include <texteditor/textmark.h>
 #include <texteditor/typingsettings.h>
 
+#include <utils/algorithm.h>
 #include <utils/aspects.h>
 #include <utils/fancylineedit.h>
 #include <utils/hostosinfo.h>
@@ -944,9 +945,8 @@ public:
             QString found = sel.selectedText();
             // Only add "real" completions.
             if (found.startsWith(needle)
-                    && !seen.contains(found)
-                    && sel.anchor() != basePosition) {
-                seen.insert(found);
+                    && sel.anchor() != basePosition
+                    && Utils::insert(seen, found)) {
                 auto item = new FakeVimAssistProposalItem(m_provider);
                 item->setText(found);
                 items.append(item);