Utils: Replace FilePath part setters

... by a combined version. This will make it easier to store the parts
in one QString object.

Change-Id: Ie85a77e3957c78a30e49998fe2e617af35a8ad17
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/libs/qmljs/qmljsmodelmanagerinterface.cpp

@@ -1200,7 +1200,7 @@ static QList<Utils::FilePath> minimalPrefixPaths(const QList<Utils::FilePath> &p
     // find minimal prefix, ensure '/' at end
     for (Utils::FilePath path : qAsConst(paths)) {
         if (!path.endsWith("/"))
-            path.setPath(QString(path.path() + "/"));
+            path = path.withNewPath(path.path() + "/");
         if (path.path().length() > 1)
             sortedPaths.append(path);
     }