FilePath: Return QString copy from toString()

Returning a const reference is different from our usual coding style,
and can lead to crashes when assigning to a const reference, like

const QString &path = filePath.pathAppended("foo").toString();

Make it behave like our other API.

Change-Id: Iab1cf1a944be405227b135c12029f58869893911
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/libs/utils/fileutils.h

@@ -78,7 +78,7 @@ public:
     static FilePath fromUtf8(const char *filepath, int filepathSize = -1);
     static FilePath fromVariant(const QVariant &variant);
 
-    const QString &toString() const;
+    QString toString() const;
     QFileInfo toFileInfo() const;
     QVariant toVariant() const;